force the browse dialog to only accept images...

...on image upload.

This won't prevent the issue reported in #773 but it will at least provide a better experience for users as only images will show up in the dialog.

More information on the `accept` attribute available on [wufoo](http://www.wufoo.com/html5/attributes/07-accept.html).

**Note:** IE9 will take the attribute but won't filter.