Improvements to FileCache logic.

[pixlwave]

Whenever we download media to show via AVPlayer/QuickLook the file is cached using naive guesses of the file extension. As we're writing Data to disk, the file doesn't have any useful type metadata, so if the file extension is wrong (e.g. #443) then playback/previewing will fail. We should either:

• Add more robust file type detection by reading the mimetype from the event, and then checking the data received.
• Pass this over to Rust itself and instead have it return us a valid URL to open the file from rather than data which we write to disk.

[stefanceriu]

Relates to #316