Extend AttachmentProvider to be able to get "raw" attachment

AttachmentProvider can now return an attachment with an unmodified
MIME type. However, when viewing attachments this is not desirable. So
we try hard to return a sensible MIME type (use extension to look up
a MIME type if it's originally "application/octet-stream"; or replace
with canonical MIME type if it's known to be wrong, e.g.
"image/jpg" -> "image/jpeg")