improve detection of ajax request

improve detection of ajax request

Firefox sent an accept header for application/xml, not application/json,
and also weakened the priority to 0.8. So that stuff is not to be trusted;
instead I found a better way: When an ajax upload is *not* being made,
the Upload Attachment button will be used, so enable ajax if an upload
is being made without that button having been used.

Also, testing with firefox revealed it refused to process a response that
was type application/json, and checking the demo page for the jquery file
upload plugin, it actually returns the json with type text/html. Ugh.
Followed suite.

Now tested with: chromium, chromium (w/o js), firefox, firefox (w/o js),
and w3m.