Once upon a time, for uploading file attachments and diffs to Review Board, we

would make a request to the API by sticking a form into an iframe and

submitting it. This required an ugly hack on the API side to return the

resulting response with a text/plain mimetype, so that the browser wouldn't

suggest downloading the response. This was extra confusing because the comment

with this code was incorrect, leading one to believe that XMLHttpRequest was

the problem rather than submitting HTML forms.
Having this fallback isn't too much of a problem for most cases, but it was

breaking the response parsing in the RBTools Python API. We've long since

replaced this with code that uses XMLHttpRequest, and so we can get rid of this

kludge and return the correct mimetype.

Sent a file using the RBTools Python API and saw that the response was parsed

  correctly as an ItemResource.
Uploaded file attachments via the "Update > Add File" form, drag-and-drop,

  and rbt attach.
Uploaded diffs via "Update > Update Diff" and rbt post.