extended mime-type mapping for .pptx files

mime-magic "detects" Microsoft PowerPoint 2007/2010 OpenXML files as Microsoft PowerPoint 97 files.
Sure - WHY NOT! :hankey:
Merge pull request #69 from ucsflibtopfstedt/rb2.2

various fixes and improvements
Merge pull request #70 from ilios/rb2.2

reintegrate release branch