Fall back to showing all files for unknown mime types.

It's better to let the user select among all files than not presenting
him any files at all.

Patchset 2: simplify the logic
Patchset 3: Revert to patchset 1. Verify mime type regexp name instead of mime type name.

Change-Id: I372f00cccce1c9bb69d5d2668bb33c7882580e08