Hello,

How can I view such MIME parts in-line?

The manual says:

  If builtin viewers can not display the object, Gnus resorts to
  external viewers in the ‘mailcap’ files.

So I created a ~/.mailcap file (just for testing):
--8<---------------cut here---------------start------------->8---
application/pdf;           pdftotext %s -; copiousoutput # no problem here
application/octet-stream;  pdftotext %s -; copiousoutput # does not work
--8<---------------cut here---------------end--------------->8---

When a pdf attachment has the content type `application/pdf', there is
no problem, but if it's `application/octet-stream', then emacs wants to
save the file (`Save MIME part to ...'), when pressing RET.

Please find attached 2 pdf files, one with content type
`application/pdf' and the other with `application/octet-stream' as
example.

TIA for any help!

-- 
           Peter