Finder Enhancement - Customize Icon for filetype
This works for PDFs, and many other types of documents because the OS X Quick Look Framework can natively read those filetypes. All other filetypes get just an icon rather than a preview. Best guess is that that framework doesn't support .mobi files.

Without native support from Apple, your best bet for .mobi (and any other unsupported file format), is to look for a plugin. A quick Google search turned up this. I don't have any .mobi files to test it on, but it's worth a shot.

Generic QuickLook Plugin installation guide here. Good luck!