Fix issue when uploading files with firefox over the Document AddForm

Added temporary monkeypatch to ignore mimetypes sent by the browser (see https://github.com/plone/plone.formwidget.namedfile/pull/9):
We can't include the fix with the newests version (1.0.11), because this versions need also the newest z3c.form (see https://github.com/plone/plone.formwidget.namedfile/pull/10).

With the planed update to Plone 4.3 this monkeypatch should be removed.

@lukasgraf please have a look ...
Shouldn't be keep all monkey patches in `opengever/base/monkeypatch.py`? Makes it much easier to go through them and check if we still need them.