Shimmie2

There should be a simple way for one to specify an image that's tied to a flash file or an SVG file that will override the default thumb.jpg file. Here's the way I'd imagine it:

When you first upload a flash file (or some other un-thumbnailable file), the default thumbnail will be assigned (thumb.jpg). Next, when you view the file there would be a spot underneath the Tags and Source boxes that says "Custom Thumbnail" and an upload box. When you upload this new file, it will replace the default thumbnail with a custom one.

As for MP3 files, just have a mini-player in place of the thumbnail and then a link below to go to the /post/view/ page.