Adding more office MIME types to office_converter.

- Without those types the graphics backend will throw 'Unkwon file format' on OCR.
- S.a.
  http://stackoverflow.com/questions/4212861/what-is-a-correct-mime-type-for-docx-pptx-etc
Merge pull request #43 from mbehrle/feature/mimetypes

Adding more office MIME types to office_converter.