Mime pdf - opened in external viewer

[Thorsten]

Hi List,
I want to send an email with a pdf file attached. I attached the file
with C-c RET f and then try to send the message with C-c C-c (or C-c
C-s), but the only thing that happens is that the attached file is
opened in my pdf-viewer and gnus shows the message:
"<file> opened in external viewer"
but the message is never send, even if I repend the send command several
times. 

I did not find the solution in the manual or a related question in the
newsgroup, therefore this probably quite basic question.
Thanks for any help
Thorsten