convert mime type 'application/octet' into pdf files

[rsoika]

During the extraction of file attachements of an email it can happen, that a attachement with file extension '.pdf' has the mimetype is application/octet . The current implementation does not convert the mime type. This is only done for application/octet-stream

We need to extend this logic of converting octet mime types.

As a result currently the web application did not open these pdf files with the inline viewer.

see also issue #147