fix file upload in overlays

To include files in our upload this uses the FormData API.

Unfortunately, FormData is only available in IE 10+. In
older IEs, the whole form will be unusable.

To limit the damage this does only use FormData where necessary.
use FormData whenever available

This is an improvement because now forms with
enctype="multipart/form-data" will non break completely
on non-supporting browsers
add warning to ajax form submit for old browsers
Merge pull request #728 from xi/feature-724-fix-file-upload-in-overlay

fix file upload in overlays