I always thought that the .mpf file extension was a proprietary file format used with Microsoft's Clip Organizer product or via their online clipart gallery. In fact, it's not very exciting or difficult to extract clip art (or I assume other media) from the package file, since it's only an xml file with the clipart files encoded as Base64 inside.
I have a legal copy of the Clip Organizer product. However, I don't necessarily install it (nor want to) on every computer. But, I do make extensive use of Microsoft's online clipart gallery which means I get a lot of ClipArt.mpf files lying around.
Searching, I found a PERL script to extract mpf files. But, I don't use Perl regularly, so getting the right perl modules installed wasn't exciting. I decided it would just be easier to write a parser / extractor in Java. If you're interested, here is the (free) java source code:
The class requires the Apache Commons Codec library to be in the classpath.