Fix for Content-Disposition encoding.

src/Core/Extensions/Http.cs

@@ -85,7 +85,7 @@ private static HttpRequestMessage CreateProxiedHttpRequest(this HttpContext cont
                 if (request.HasFormContentType)
                 {
                     usesStreamContent = false;
-                    requestMessage.Content = request.Form.ToHttpContent(request.ContentType);
+                    requestMessage.Content = request.Form.ToHttpContent(request);
                 }
                 else
                 {

src/Core/Helpers/Helpers.cs

@@ -3,6 +3,7 @@
 using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
+using System.Text;
 using System.Threading.Tasks;
 using System.Web;
 using Microsoft.AspNetCore.Http;
@@ -72,7 +73,7 @@ internal static string TrimTrailingSlashes(this string s)
             return s.Substring(0, s.Length - count);
         }
 
-        internal static HttpContent ToHttpContent(this IFormCollection collection, string contentTypeHeader)
+        internal static HttpContent ToHttpContent(this IFormCollection collection, HttpRequest request)
         {
             // @PreferLinux:
             // Form content types resource: https://stackoverflow.com/questions/4526273/what-does-enctype-multipart-form-data-mean/28380690
@@ -84,7 +85,7 @@ internal static HttpContent ToHttpContent(this IFormCollection collection, strin
             // A single form element can have multiple values. When sending them they are handled as separate items with the same name, not a singe item with multiple values.
             // For example, a=1&a=2.
 
-            var contentType = MediaTypeHeaderValue.Parse(contentTypeHeader);
+            var contentType = MediaTypeHeaderValue.Parse(request.ContentType);
 
             if (contentType.MediaType.Equals("application/x-www-form-urlencoded", StringComparison.OrdinalIgnoreCase)) // specification: https://url.spec.whatwg.org/#concept-urlencoded
                 return new FormUrlEncodedContent(collection.SelectMany(formItemList => formItemList.Value.Select(value => new KeyValuePair<string, string>(formItemList.Key, value))));
@@ -106,9 +107,15 @@ internal static HttpContent ToHttpContent(this IFormCollection collection, strin
             foreach (var file in collection.Files)
             {
                 var content = new StreamContent(file.OpenReadStream());
-                foreach (var header in file.Headers)
+                foreach (var header in file.Headers.Where(h => !h.Key.Equals("Content-Disposition", StringComparison.OrdinalIgnoreCase)))
                     content.Headers.TryAddWithoutValidation(header.Key, (IEnumerable<string>)header.Value);
-                multipart.Add(content, file.Name, file.FileName);
+
+                // Force content-disposition header to use raw string to ensure UTF-8 is well encoded.
+                content.Headers.TryAddWithoutValidation("Content-Disposition",
+                    new string(Encoding.UTF8.GetBytes($"form-data; name=\"{file.Name}\"; filename=\"{file.FileName}\"").
+                    Select(b => (char)b).ToArray()));
+
+                multipart.Add(content);
             }
             return multipart;
         }

src/Test/Http/HttpIntegrationTests.cs

@@ -79,7 +79,7 @@ public async Task CanProxyControllerPostWithFormAndFilesRequest()
             const string fileString = "This is a test file こんにちは with non-ascii content.";
             var fileContent = new StreamContent(new System.IO.MemoryStream(Encoding.UTF8.GetBytes(fileString)));
             content.Add(fileContent, "testFile", fileName);
-            
+
             var response = await _client.PostAsync("api/multipart", content);
             response.EnsureSuccessStatusCode();
             var responseString = await response.Content.ReadAsStringAsync();

src/Test/Unit/Other.cs

@@ -1,5 +1,7 @@
 
 using System;
+using Microsoft.AspNetCore.Http;
+using Moq;
 using Xunit;
 
 namespace AspNetCore.Proxy.Tests
@@ -10,9 +12,11 @@ public class Other
         public void CanFailOnBadFormContentType()
         {
             var contentType = "text/plain";
+            var request = Mock.Of<HttpRequest>();
+            request.ContentType = contentType;
 
             var e = Assert.ThrowsAny<Exception>(() => {
-                var dummy = Helpers.ToHttpContent(null, contentType);
+                var dummy = Helpers.ToHttpContent(null, request);
             });
 
             Assert.Equal($"Unknown form content type `{contentType}`.", e.Message);