[BUG] all OCR mimetypes forced to start with text, excluding mimetype of application/xml for hOCR

[rosiel]

What steps does it take to reproduce the issue?

• When does this issue occur?
  When trying to set up an action to extract hocr

• Which page does it occur on?
  http://localhost:8000/admin/config/system/actions/add/generate_ocr_derivative

• What happens?
  Cant' enter anything but a text/foo mimetype

• To whom does it occur (anonymous visitor, editor, administrator)?

• What did you expect to happen?

Which version of Islandora are you using?

2.8.0

Any related open or closed issues to this bug report?

Screenshots:

[rosiel]

Fixed by 5e958a5