Since https://github.com/w3c/FileAPI/issues/43 is still open, it is unclear how body.type should work.
The current wpts expect some behavior which isn't in the specifications.
So, since the situation is very messy in the specifications, the patch is doing a
spot fix for boundary handling. It is ugly, but shouldn't change other behavior.
Differential Revision: https://phabricator.services.mozilla.com/D150018
