Special file handling

[xfbs]

We should handle some file types specially:

• If you look at a Markdown file, you should be able to render it as an actual markdown file (we could still show a diff, but when you render it, it would be word-based and not line-based)
• If you look at an image/pdf/..., we should display it
• If you look at a binary file, we should show it as hex-encoded (similar to xxd or hexdump output)