MimePartMatcher does not match multipart/form-data request body (possible bug?)

[BrunoBeraud]

Hello,
First of all, thank you for this library — it helps us a lot in our development work!

I'm using WireMock.Net.Testcontainers 1.15.0 for our tests and the sheyenrath/wiremock.net-alpine:1.15.0 Docker image in our development environment.
We want to define a stub endpoint that accepts a multipart/form-data HTTP request.

Unfortunately, we’re not able to create a request body matcher using MimePartMatcher in a JSON mapping file.

---

What Works

This body matcher works correctly:

"Body": {
  "Matcher": {
    "Name": "RegexMatcher",
    "Pattern": ".+"
  }
}

---

What Doesn’t Work

When using MimePartMatcher, even with very lenient matchers, the matcher always returns a 0.0 score for the body:

"Body": {
  "Matcher": {
    "Name": "MimePartMatcher",
    "ContentTypeMatcher": {
      "Name": "RegexMatcher",
      "Pattern": ".*"
    },
    "ContentDispositionMatcher": {
      "Name": "RegexMatcher",
      "Pattern": ".*"
    },
    "ContentMatcher": {
      "Name": "RegexMatcher",
      "Pattern": ".*"
    }
  }
}

I also tried:

• WildcardMatcher with * for all matchers
• JsonMatcher with the exact JSON sent
• JsonPartialMatcher using regex (with "Regex": true) and .* for all JSON values
• Setting "ContentTypeMatcher" with "Name": "ContentTypeMatcher" and "Pattern": "application/json"

→ Always a 0.0 score for BodyMatcher (while PathMatcher, MethodMatcher, and HeadersMatcher all score 1.0).

---

Request Example

Here’s a sample curl request (exported from Postman):

curl -L -X POST "http://localhost:8080/my-stub-endpoint" \
  -F "metadata=\"{\\\"ID\\\": \\\"9858013b-e020-4ef9-b8a8-0bebc740e6a7\\\", \\\"DATE\\\": \\\"2025-08-15T13:45:30.0000000Z\\\", \\\"NAME\\\": \\\"32c9a8dd-e214-4afb-9611-9cde81f827c6\\\", \\\"NUMBER\\\": 10}\";type=application/json"

---

Observation

From the /__admin/requests endpoint, here’s an example of the request body WireMock receives:

"Headers": {
        "Accept": [
          "*/*"
        ],
        "Host": [
          "localhost:8080"
        ],
        "User-Agent": [
          "curl/8.13.0"
        ],
        "Content-Type": [
          "multipart/form-data; boundary=------------------------woli8b80pw4vBJtNpAMOKS"
        ],
        "Content-Length": [
          "334"
        ]
      },
"Body": "--------------------------woli8b80pw4vBJtNpAMOKS\r\n
Content-Disposition: form-data; name=\"metadata\"\r\n
Content-Type: application/json\r\n
\r\n
{\"ID\": \"9858013b-e020-4ef9-b8a8-0bebc740e6a7\", \"DATE\": \"2025-08-15T13:45:30.0000000Z\", \"NAME\": \"32c9a8dd-e214-4afb-9611-9cde81f827c6\", \"NUMBER\": 10}\r\n
--------------------------woli8b80pw4vBJtNpAMOKS--\r\n"

After digging a bit into the code, it seems that the issue comes from the use of the MimeKit library, which appears unable to parse this request body.
As a result, WireMock fails to extract the body parts, and the matcher score stays at its default value (0.0).

---

Possible Root Cause

It looks like the body content doesn’t include headers (such as Content-Type with a boundary) that would allow MimeKit to properly parse it into BodyParts.
In real HTTP requests, this header is not part of the body — it’s in the request headers — which might explain the mismatch compared to the examples in MimePartMatcherTests, where the content type is included in the body.

---

Question

Could this be a limitation or a bug in how MimePartMatcher / MimeKit handles real multipart/form-data requests?
Or is there a recommended approach to match multipart requests in JSON mappings?

Thank you in advance for your time and for maintaining this great library!

[StefH]

@BrunoBeraud
I need to check.

[StefH]

@BrunoBeraud
I did check the code and it's correct, in my unit tests I need to pass that text (including Content-Type as part of the body) to get a valid MimeMessage with 3 parts.

The real code will use the Header and prepend this to your body.

---

I think that your mapping json is wrong.
A possible mapping json for checking 3 parts (same as in the unit test) can be like:

{
    "Guid": "b9c82182-e469-41da-bcaf-b6e3157fefdb",
    "UpdatedAt": "2023-07-24T18:12:55.564978Z",
    "Request": {
        "Path": {
            "Matchers": [
                {
                    "Name": "WildcardMatcher",
                    "Pattern": "/multipart",
                    "IgnoreCase": false
                }
            ]
        },
        "Methods": [
            "POST"
        ],
        "Body": {
            "Matchers": [
                {
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "text/plain",
                        "IgnoreCase": false
                    },
                    "ContentMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "This is some plain text",
                        "IgnoreCase": false
                    },
                    "Name": "MimePartMatcher"
                },
                {
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "text/json",
                        "IgnoreCase": false
                    },
                    "ContentMatcher": {
                        "Name": "JsonMatcher",
                        "Pattern": {
                            "Key": "Value"
                        },
                        "IgnoreCase": true
                    },
                    "Name": "MimePartMatcher"
                },
                {
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "image/png",
                        "IgnoreCase": true
                    },
                    "ContentDispositionMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "attachment; filename=\"image.png\"",
                        "IgnoreCase": true
                    },
                    "ContentTransferEncodingMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "base64",
                        "IgnoreCase": true
                    },
                    "ContentMatcher": {
                        "Name": "ExactObjectMatcher",
                        "Pattern": "iVBORw0KGgoAAAANSUhEUgAAAAIAAAACAgMAAAAP2OW3AAAADFBMVEX/tID/vpH/pWX/sHidUyjlAAAADElEQVR4XmMQYNgAAADkAMHebX3mAAAAAElFTkSuQmCC"
                    },
                    "Name": "MimePartMatcher"
                }
            ],
            "MatchOperator": "Or"
        }
    },
    "Response": {
        "BodyDestination": "SameAsSource",
        "Body": "MultiPart is ok"
    }
}

---

See also this extra unit test --> #1389

[BrunoBeraud]

Hello, I have tried your json multipart request mapping file with sheyenrath/wiremock.net-alpine:1.15.0 docker container.
I used this curl request:
curl -X POST http://localhost:8080/multipart \ -F 'plainText=This is some plain text;type=text/plain' \ -F 'jsonData={ "Key": "Value" };type=text/json' \ -F 'image=@image.png;type=image/png'
The image is the one expecting in the ExactObjectMatcher base64 pattern

MatchDetails all 1.0 but BodyMatcher still 0.0...

[StefH]

@BrunoBeraud

I did not test the Docker image, but just local code.
And the mapping in json is like:

{
    "Guid": "b9c82182-e469-41da-bcaf-b6e3157fefdb",
    "UpdatedAt": "2025-12-18T17:21:57.3879723Z",
    "Request": {
        "Path": {
            "Matchers": [
                {
                    "Name": "WildcardMatcher",
                    "Pattern": "/multipart",
                    "IgnoreCase": false
                }
            ]
        },
        "Methods": [
            "POST"
        ],
        "Body": {
            "Matchers": [
                {
                    "Name": "MimePartMatcher",
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "text/plain",
                        "IgnoreCase": false
                    },
                    "ContentMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "This is some plain text",
                        "IgnoreCase": false
                    }
                },
                {
                    "Name": "MimePartMatcher",
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "text/json",
                        "IgnoreCase": false
                    },
                    "ContentMatcher": {
                        "Name": "JsonMatcher",
                        "Pattern": {
                            "Key": "Value"
                        },
                        "IgnoreCase": true,
                        "Regex": false
                    }
                },
                {
                    "Name": "MimePartMatcher",
                    "ContentTypeMatcher": {
                        "Name": "ContentTypeMatcher",
                        "Pattern": "image/png",
                        "IgnoreCase": false
                    },
                    "ContentDispositionMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "attachment; filename=\"image.png\"",
                        "IgnoreCase": false
                    },
                    "ContentTransferEncodingMatcher": {
                        "Name": "ExactMatcher",
                        "Pattern": "base64",
                        "IgnoreCase": false
                    },
                    "ContentMatcher": {
                        "Name": "ExactObjectMatcher",
                        "Pattern": "iVBORw0KGgoAAAANSUhEUgAAAAIAAAACAgMAAAAP2OW3AAAADFBMVEX/tID/vpH/pWX/sHidUyjlAAAADElEQVR4XmMQYNgAAADkAMHebX3mAAAAAElFTkSuQmCC"
                    }
                }
            ],
            "MatchOperator": "Or"
        }
    },
    "Response": {
        "BodyDestination": "SameAsSource",
        "Body": "MultiPart is ok"
    }
}

The same mapping in code is:

var textPlainContentTypeMatcher = new ContentTypeMatcher("text/plain");
var textPlainContentMatcher = new ExactMatcher("This is some plain text");
var textPlainMatcher = new MimePartMatcher(MatchBehaviour.AcceptOnMatch, textPlainContentTypeMatcher, null, null, textPlainContentMatcher);

var textJsonContentTypeMatcher = new ContentTypeMatcher("text/json");
var textJsonContentMatcher = new JsonMatcher(new { Key = "Value" }, true);
var textJsonMatcher = new MimePartMatcher(MatchBehaviour.AcceptOnMatch, textJsonContentTypeMatcher, null, null, textJsonContentMatcher);

var imagePngContentTypeMatcher = new ContentTypeMatcher("image/png");
var imagePngContentDispositionMatcher = new ExactMatcher("attachment; filename=\"image.png\"");
var imagePngContentTransferEncodingMatcher = new ExactMatcher("base64");
var imagePngContentMatcher = new ExactObjectMatcher(Convert.FromBase64String("iVBORw0KGgoAAAANSUhEUgAAAAIAAAACAgMAAAAP2OW3AAAADFBMVEX/tID/vpH/pWX/sHidUyjlAAAADElEQVR4XmMQYNgAAADkAMHebX3mAAAAAElFTkSuQmCC"));
var imagePngMatcher = new MimePartMatcher(MatchBehaviour.AcceptOnMatch, imagePngContentTypeMatcher, imagePngContentDispositionMatcher, imagePngContentTransferEncodingMatcher, imagePngContentMatcher);

var matchers = new IMatcher[]
{
    textPlainMatcher,
    textJsonMatcher,
    imagePngMatcher
};

server
    .Given(Request.Create()
        .WithPath("/multipart")
        .UsingPost()
        .WithMultiPart(matchers)
    )
    .WithGuid("b9c82182-e469-41da-bcaf-b6e3157fefdb")
    .RespondWith(Response.Create()
        .WithBody("MultiPart is ok")
    );

See also:
https://github.com/wiremock/WireMock.Net/blob/mime-part-matcher/examples/WireMock.Net.Console.NET8/MainApp.cs#L559

---

The image.png is attached to this comment.
(It's a small red dot below...)

---

Running CURL on windows:

PS C:\temp> curl -X POST http://localhost:9091/multipart \ -F 'plainText=This is some plain text;type=text/plain' \ -F 'jsonData={ "Key": "Value" };type=text/json' \ -F 'image=@image.png;type=image/png'
MultiPart is okcurl: (3) URL rejected: Bad hostname
curl: (3) URL rejected: Bad hostname
curl: (3) URL rejected: Bad hostname
PS C:\temp>

---

Logging:

18/12/2025 17:30:39 [DebugRequestResponse] : Admin[False] {
  "Guid": "950acde0-f0eb-4810-ba07-022e7305c39e",
  "Request": {
    "ClientIP": "::1",
    "DateTime": "2025-12-18T17:30:37.2241528Z",
    "Path": "/multipart",
    "AbsolutePath": "/multipart",
    "Url": "http://localhost:9091/multipart",
    "AbsoluteUrl": "http://localhost:9091/multipart",
    "ProxyUrl": null,
    "Query": {},
    "Method": "POST",
    "HttpVersion": "1.1",
    "Headers": {
      "Accept": [
        "*/*"
      ],
      "Host": [
        "localhost:9091"
      ],
      "User-Agent": [
        "curl/8.16.0"
      ],
      "Content-Type": [
        "multipart/form-data; boundary=------------------------5kocnZUaqTqJbcOKOCaG3m"
      ],
      "Content-Length": [
        "591"
      ]
    },
    "Cookies": {},
    "Body": null,
    "BodyAsJson": null,
    "BodyAsBytes": "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",
    "BodyEncoding": null,
    "DetectedBodyType": "Bytes",
    "DetectedBodyTypeFromContentType": "MultiPart"
  },
  "Response": {
    "StatusCode": null,
    "Headers": {},
    "BodyDestination": "SameAsSource",
    "Body": "MultiPart is ok",
    "BodyAsJson": null,
    "BodyAsBytes": null,
    "BodyAsFile": null,
    "BodyAsFileIsCached": null,
    "BodyOriginal": null,
    "BodyEncoding": {
      "CodePage": 65001,
      "EncodingName": "Unicode (UTF-8)",
      "WebName": "utf-8"
    },
    "DetectedBodyType": 1,
    "DetectedBodyTypeFromContentType": null,
    "FaultType": null,
    "FaultPercentage": null
  },
  "MappingGuid": "b9c82182-e469-41da-bcaf-b6e3157fefdb",
  "MappingTitle": null,
  "RequestMatchResult": {
    "TotalScore": 3.0,
    "TotalNumber": 3,
    "IsPerfectMatch": true,
    "AverageTotalScore": 1.0,
    "MatchDetails": [
      {
        "Name": "PathMatcher",
        "Score": 1.0
      },
      {
        "Name": "MethodMatcher",
        "Score": 1.0
      },
      {
        "Name": "MultiPartMatcher",
        "Score": 1.0
      }
    ]
  },
  "PartialMappingGuid": "b9c82182-e469-41da-bcaf-b6e3157fefdb",
  "PartialMappingTitle": null,
  "PartialRequestMatchResult": {
    "TotalScore": 3.0,
    "TotalNumber": 3,
    "IsPerfectMatch": true,
    "AverageTotalScore": 1.0,
    "MatchDetails": [
      {
        "Name": "PathMatcher",
        "Score": 1.0
      },
      {
        "Name": "MethodMatcher",
        "Score": 1.0
      },
      {
        "Name": "MultiPartMatcher",
        "Score": 1.0
      }
    ]
  }
}