File Upload Angular2 via multipart/form-data, 400 Error - angularjs

Question

I need to upload a file together with some meta info via multipart/form-data like this:
Header
----boundary
Meta
----boundary
File
----boundary
For some reason I always get a 400 Error, the Backend server tells me, that the Meta-Field is missing. However, if I look into the Hex/Ascii dump with Wireshark, the field is definitely there.
The following Curl-command works perfectly fine and the file gets uploaded successfully:
curl -H "Expect:" -v -i -X POST -H "Content-Type: multipart/form-data" -H "Authorization: "token" -F "File=#/location" -F 'Meta={"access":"token"}' http://path
Therefore this doesn't seem to be a Backend failure. My Angular (4.1.3) request must be bad, but I can't figure out what's wrong with it.
Template-Code:
<input #file_input type="file" (change)="onFileInputChange($event)" multiple>
Angular2-Code:
onFileInputChange(event) {
let fileList: FileList = event.target.files;
for (let i = 0; i < fileList.length; i++) {
let fp: File = fileList[i];
let formData: FormData = new FormData();
formData.append('File', fp, fp.name);
const body = JSON.stringify({
'access': "token"
});
this.formData.append('Meta', body);
let RequestHeader = new Headers();
// auto set content type
RequestHeader.append('Content-Type', '');
RequestHeader.append('Accept', 'application/json');
RequestHeader.append('Authorization', "token");
this.http.post(this.backend_url, formData, RequestHeader).map(
res => res.json()).subscribe(
data => console.log('success'),
error => console.log(error))
}
}
What am I missing here?

Answer 1

I finally found the solution.
The Content-Type header was malformed:
Content-Type: , multipart/form-data; boundary=----WebKit12345
The content type header in the actual code was preset.
By calling
headers.set('Content-Type', '');
I assumed, that the header content type was overwritten, but actually an empty string was prepended to it, so that there was a comma, which could not be parsed, of course.
I solved this by deleting the content type header completely:
headers.delete('Content-Type');