Claws Mail does not set the proper MIME type for some filetypes, such as PDFs.
When I attach a PDF file to email, it is attached with MIME type application/octet-stream instead of the correct application/pdf.
The same happens with ZIP files and some other files. I suggest that claws implements correct labeling of at least the media types listed on http://www.iana.org/assignments/media-types
Some nice "background reading" on Wikipedia: https://en.wikipedia.org/wiki/Internet_media_type
1) Create a new email message.
2) Add a new attachment of type PDF.
3) Observe that it is identified as MIME type application/octet-stream instead of application/pdf.