MIME:  In looking up extensions, drop the encoding info.

E.g. for 'image/jpg;base64' we should lookup 'image/jpg'.
Shared: Fixed bug in openURL with data: URIs.

Previously the base-64 encoded bytestring was returned.
We now decode it so it's a proper image!

This should fix parsing of data: URLs.
Docx writer:  Use mime type info returned by fetchItem.