Replace \f with &#xc; in OCR output

Form Feed character causes XML validation errors.
changed str_replace to preg_replace
Merge pull request #617 from sheepeeh/master

Replace invalid characters in omeka-xml output.
Add file thumbnails to edit item form.
Fix mimetype leaking between files.

Files uploaded at the same time when validation was disabled would all
share the type of the first file.
Merge branch 'master' into derivatives