Fixes extractor worker for files with unkown mime type
Uses libmagic to guess mime type

Since we already depend on libmagic we can stop relying on file extension to
guess the mimetype of the file.
Handle the file in it's original encoding if libmagic doesn't detect a valid one
Returns all expected data from worker even if mimetype is not supported
Merge pull request #136 from flavioamieiro/bugfix/unkown_mimetype_or_encoding

Bugfix/unkown mimetype or encoding