Fixed image gallery filesize warning #2869

There were 2 file size validators present.  The first printed a warning, but didn't validate.  The second wasn't available for a warning, but did validate.  I updated the first to show the smaller of the two filesizes (the default php_max_upload or the field's 6mb).
Merge pull request #4989 from openscholar/2869-gallery_size

Fixed image gallery filesize warning #2869