I have looked further into the issue now. It seems those bytes are appended to the refresh-Link URL which is called by the Uploadify SWF when the upload has finished.
http://localhost/ss245_new/admin/EditForm/field/GalleryItems/UploadifyForm/field/UploadedFiles/refresh?SecurityID=56790d566e0a5692041b48af59f6f745c175090d&ctf[GalleryItems][start]=0&ctf[GalleryItems][per_page]=10&ctf[GalleryItems][showall]=0&ctf[GalleryItems][sort]=SortOrder&ctf[GalleryItems][sort_dir]=&ctf[GalleryItems][search]=&ctf[GalleryItems][filter]=AlbumID_4&ctf[GalleryItems][view]=grid&ctf[GalleryItems][imagesize]=100&FileIDs=%EF%BB%BF320
You can see the FileIDs field has those strange hex-codes before the id of the uploaded image, which would be 320 in this case. I'm currently trying to figure out where these come from.