Various updates needed for this add-on

Hello,

this add-on has various problems:

It depends on bootstrap.py which tends to date very fast. Better replace it with virtualenv and requirements.cfg (similar to buildout.coredev)
freedesktop.org.xml is outdated. Additionally when using the latest one, the parsing breaks.
Tests can be improved (add relevant cases, remove irrelevant cases, test fixture bases on plone.app.contenttypes without being necessary)

I am planning to address the aforementioned problems with an upcoming PR and that's why I created this issue.

plone / Products.MimetypesRegistry