If the server returns the MIME type as application/octet-stream we
assume it doesn't really know what the MIME type. This patch tries
matching the MIME type from the file extension instead in this case.
This enables the use of servers (like OneDrive for Business) which
don't allow the setting of MIME types on upload and have a poor
selection of mime types.
Fixes#7259
Nick Craig-Wood
85c29e3629