Video thumbnail files do not have .jpg extension

When using mplayer on windows to create video thumbnails, the jpgs get saved on the file system without an appropriate extension. The '_small' and '_thumb' files generated from videos had no extension, but were valid jpg files. This behavior did not affect the functioning of the website; the thumbnails still showed up just fine in the webpage....

The problem is that the thumbnail filenames are created using the video class (because it is a video node), when in fact the images created using mplayer are jpgs. So, when getting an appropriate filename extension, the 'photo' class should be used when creating the filename. See attached patch.