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 ...
As we discussed, I'll move them to the `base/monkeypatch.py`. 

But we should check and split them up in to separate files and a move it to a own `monkeypatch` directory, when updating to newest Plone version.