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 ...