correct content-type for webp 

@drsnyder 
Adding custom-mimetypes.xml in the correct place for Tika to read it ([link](http://tika.apache.org/1.0/parser_guide.html#Add_your_MIME-Type)). I'll also submit a patch to them to add to their core mimetypes xml. [JIRA](https://wikia-inc.atlassian.net/browse/PLATFORM-518) so it doesn't get lost/forgotten.

Modeled after other RIFF-spec mime-type checkers (search for RIFF....WAVE [here](http://svn.apache.org/repos/asf/tika/trunk/tika-core/src/main/resources/org/apache/tika/mime/tika-mimetypes.xml))