Add extension->mime type mappings

* Microsoft Office common extensions
* .xml -> text/xml
* Change existing mapping of ".pot"->"text/plain" to ".po"

Note:  We already had a definition for .pot to text/plain.  Per the
information found at <http://www.fileinfo.com/extension/pot> this is the
rarely seen "portable object file", and is often named ".po" to avoid
this same conflict.

Addresses bug # 2097457

Change-Id: I964b853fdeae77d742f627c535a8b32e0e53516a