How does HTTP file upload work?

Question

When I submit a simple form like this with a file attached:

Answer 1

Send file as binary content (upload without form or FormData)

In the given answers/examples the file is (most likely) uploaded with a HTML form or using the FormData API. The file is only a part of the data sent in the request, hence the multipart/form-data Content-Type header.

If you want to send the file as the only content then you can directly add it as the request body and you set the Content-Type header to the MIME type of the file you are sending. The file name can be added in the Content-Disposition header. You can upload like this:

var xmlHttpRequest = new XMLHttpRequest();

var file = ...file handle...
var fileName = ...file name...
var target = ...target...
var mimeType = ...mime type...

xmlHttpRequest.open('POST', target, true);
xmlHttpRequest.setRequestHeader('Content-Type', mimeType);
xmlHttpRequest.setRequestHeader('Content-Disposition', 'attachment; filename="' + fileName + '"');
xmlHttpRequest.send(file);

If you don't (want to) use forms and you are only interested in uploading one single file this is the easiest way to include your file in the request.