IMAPImportService - convert mime type for pdf files

[rsoika]

During the extraction of file attachements of an email it can happen, that a attached PDF File (typically with upper case file extention) is not detected as PDF type.
The mime type in these cases is application/octet-stream

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

We can fix this rare case if we test for mimetype = application/octet-stream and file extension PDF and convert the mime type to application/pdf