Change Content-Disposition Header

- Make ALLOWED_CONTENT_TYPES public in order to use
- Add $fileName variable which uses "$node->getName()" to get the proper file name and "$file->getMimeType()" along with the ALLOWED_CONTENT_TYPES array in PhotoCache.php to get the proper file extension
- Make "$fileName" the name of the file in the Content-Disposition header when downloading a Contact's photo
- Add filename to the CardDAV integration image export test header
- Change headers in ImageExportPluginTest to reflect changes

Signed-off-by: Jacob Neplokh <me@jacobneplokh.com>