I think I've got it—here's what I learned, for future reference:
(1) I probably should have tried this earlier, but clicking the "display file details" button in the uploader show the mimetype of the files you're uploading. In this case, it turns out I needed AZW/mobi. Huh.
(2) Out of desperation, I threw my same mobi.png files into the core pix/f/ folder, and the mobi icons all of a sudden appeared as expected. That didn't make sense, since my epub icons in my theme's pix_core/f/ folder were overriding the original epub icons. And then it did make sense—I'm guessing that my mobi icons in my theme's folder weren't overriding Moodle's default icon because there was no default icon to override in the first place. When I added a mobi icon to the core, everything clicked into place.
Main lesson? When adding a completely new mimetype, add the icons to the core instead of the theme. (I suppose.)