Don't use x-* MIME types when possible

[Abion47]

MIME types that have the x-* prefix are by definition non-standard extension types. Many formats that used to use these types no longer do as they have become official MIME types recognized by the IANA, but the mime package is still using the x-* mime type on many of them. This can cause problems as many APIs will support the standard form but not the non-standard extension form, so they should be updated where appropriate.

Here are some examples:

audio/x-aiff => audio/aiff
audio/x-wav => audio/wav (or) audio/vnd.wav