Autorunning a random file downloaded from very untrusty source is a great way to get trojaned. You know, your OS might be intelligent (and stupid) enough to determine, "hey, this file called hotchick.jpg is actually a perl script, so let's just execute it instead of running the picture viewer".
And regarding partial files - most apps barf on partially completed downloads. VLC being an exception, but video preview is what we already have.
Makes sense. I was actually thinking of either a hard requirement for the first and last chunks to be available or a mimetype whitelist but both would be even better. (I considered looking up the mimetype by extension but, if the first chunk isn't present, even MPlayer is liable to get confused about what filetype it's dealing with.)
It's been my experience that, as long as the first and last chunks are present, a surprising number of formats can return useful information without being complete.
For example, video and audio aside, zip files store their directory record at the end of the file, so you can get pretty much all the metadata you could ever want from a partial.
Still, I suppose that is getting a little involved.