It happens because doc-view was added to mailcap-mime-data; I am using
(defun ensc/mailcap-mime-data-filter (filter)
""
(mapcar (lambda(major)
(append (list (car major))
(remove nil
(mapcar (lambda(minor)
(when (funcall filter (car major) (car minor) (cdr minor))
minor))
(cdr major)))))
mailcap-mime-data))
(defun ensc/no-pdf-doc-view-filter (major minor spec)
(if (and (string= major "application")
(string= minor "pdf")
(member '(viewer . doc-view-mode) spec))
nil
t))
(eval-after-load 'mailcap
'(progn
(setq mailcap-mime-data
(ensc/mailcap-mime-data-filter 'ensc/no-pdf-doc-view-filter))))
to remove corresponding entry.
This does not solve only the org-mode problem but allows e.g. to display PDFs in gnus using an external viewer.
