To reproduce the bug,

    1. Install pcmanfm
    2. emacs -Q
    3. Eval the following lines 

    (defun vz/message-yank-media-copied-files-handler (_ data)
      "Attach files copied/cut from the file manager.
    DATA is a string where the first line is the operation to
    perform: copy or cut.  Rest are file: links.  The operation is
    always ignored, and the files are only attached."
      (let* ((files (cdr (split-string data "[\0\n\r]" t "^file://"))))
        (dolist (f files)
          (mml-attach-file f (or (mm-default-file-type f) "application/octet-stream")))))

     (add-hook 'message-mode-hook
               (defun vz/message-register-yank-media-handler ()
                 (yank-media-handler "x-special/gnome-copied-files"
                                     #'vz/message-yank-media-copied-files-handler)))
    4. C-x m
    5. Cut a file from pcmanfm using C-x
    6. In Emacs, choose the message-mode buffer and say M-x yank-media
    7. Witness the wrong-type-argument error

This happens because pcmanfm "wipes" the clipboard data it is accessed
once.  This is a problem because yank-media uses
yank-media--get-selection to access the clipboard data twice

    · Once in yank-media--find-matching-media to find out if the
      buffer's registered handlers can work with the data (see
      pcase-dolist form in yank-media function).  At this point,
      vz/message-register-yank-media-handler is chosen since it can
      handle x-special/gnome-copied-files (which is the type used for
      the cut file).

    · Second when yank-media calls the handler to do its thing, which
      can be seen in the funcall forms in yank-media function again.

The problem here is that once yank-media--find-matching-media calls
yank-media--get-selection to get the clipboard selection, pcmanfm wipes
the clipboard so when yank-media calls that function again and passes
the return value to the handler, the handler receives nil.

The attached patch fixes this issue by making
yank-media--find-matching-media return the selection value, and passing
the returned value to the handler avoiding calling
yank-media--get-selection again.

P.S. I don't know if it is a problem with other file managers.
P.P.S. I plan on submitting a patch to add the yank-media-handler here
       once this bug is fixed.