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")
This reverts commit 9fc94115adb58dcd8a64227a93d0e6e707d7752d.
It turns out that disabling the "View attachment" button (rather than
giving a useful error message telling the user what mime type the app
needs to support) was confusing to users.
