Add ext functions to file types

[giraffekey]

Adds two simple quality of life functions to FileType and MimeType.

FileType::Mpeg.ext(); // Some("mp3")
MimeType::Jpeg.ext(); // Some("jpg")

My use case is that I want to extract the pictures and save them separately to disk, so this saves me a manual pattern match on the extension of the file.

[Serial-ATA]

Thanks for the PR!

I think having this on MimeType is fine, but it probably shouldn't be on FileType.

[giraffekey]

Makes sense.

[Serial-ATA]

Thanks!