Update to set mimetype and added detecting mimetype

after updating requests module to 2.1.1, I couldn't upload file jobs. 
so I added mimetype into parameter and detecting function if you don't set it.
@toshipon 
```
Return value is a tuple (type, encoding) where type is None if the
type can't be guessed
```
when `type` is `None` can you please use: `application/octet-stream`