Auto merge of #12780 - izgzhen:xhr-file-upload, r=Manishearth

Add XHR support for FormData and related fixes

+ Update IDL for `FormData`, and related fixes
+ Reuse `htmlformelement::FormDatum` for `formdata.rs` for a uniform representation, and related fixes
+ Add support for `FormData` branch of XHR `BodyInit`.
+ Fixes a lot of related WPT tests

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes OR

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tests/wpt/mozilla/tests/mozilla/FileAPI/resource/file-submission.py

@@ -21,7 +21,7 @@ def main(request, response):
     boundary = content_type[1].strip("boundary=")
 
     body = "--" + boundary + "\r\nContent-Disposition: form-data; name=\"file-input\"; filename=\"upload.txt\""
-    body += "\r\n" + "text/plain\r\n\r\nHello\r\n--" + boundary + "--"
+    body += "\r\n" + "content-type: text/plain\r\n\r\nHello\r\n--" + boundary + "--"
 
     if body != request.body:
         return fail("request body doesn't match: " + body + "+++++++" + request.body)