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 Example instructions: 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.
see http://www.claws-mail.org/faq/index.php/Installation_and_Configuration#How_can_I_change_the_suggested_mimetype_for_attachments_I_am_sending.3F