If the server reports an attachment's mime type as either text/plain or
application/octet-stream, we will try to infer the real mime type using
the attachment's extension. If one cannot be found, we either synthesize
it (if original mime type is application-octet-stream), or, we use the
server specified mime type (if the original is text/plain).
bug 3379416
Change-Id: I331e767ed36e4e17756025cc816bdb7b5a8f0868
34212cc7df