Bug 895726 - [Buri][Monitor][MMS] Unable to save attachement from Camera

Continue the work from https://github.com/mozilla-b2g/gaia/pull/11747. This patch is focused on refine the behavior about viewing attachment with open activity:
 - Make sure attachment is open-able even blob mimetype is unsupported but file extension is valid.
 - Make sure attachment is savable even file extenion is missing or invalid but mimetype is supported.