fix(middleware): prevent any tries reading from the same stream twice (…

…#2)

BREAKING CHANGE: `stream` property is replaced with `toStream` method that allows the user to create a new readable stream from the temp file each time.

lib/middleware.js

@@ -1,5 +1,5 @@
 const is = require('type-is')
-const { createReadStream, unlink } = require('fs')
+const { createReadStream } = require('fs')
 const appendField = require('append-field')
 const { createFileAppender } = require('./file-appender')
 const readBody = require('./read-body')
@@ -17,9 +17,7 @@ const handleRequest = async (setup, req) => {
   const fileAppender = createFileAppender(options.fileStrategy, req, options.fields)
 
   for (const file of result.files) {
-    file.stream = createReadStream(file.path)
-    file.stream.once('open', () => unlink(file.path, () => {}))
-
+    file.toStream = () => createReadStream(file.path)
     fileAppender.append(file)
   }
 }