Open the directory in dired with C-x 4 d and type M-x turn-on-gnus-dired-mode. Then mark the required files with m (or whatever) and type C-c RET C-a. The files can now be attached to an already opened message buffer or assigned to a new one.
e runs the command mu4e-view-save-attachment
Offer to save attachment(s). If MULTI (prefix-argument) is nil, save a
single one, otherwise, offer to save a range of attachments.
Alternatively, with point on an attachment name, you can use S-RET:
S-return runs the command mu4e~view-save-attach-from-binding
Save the attachement at ...
I know nothing about mail in Emacs, but running C-h k C-x m tells me that C-x m runs the command compose-mail, which is different from mail. This is presumably why they behave differently. You can find out more with C-h f compose-mail and C-h f mail.
Org-mode files are plain text, by design, and Org therefore has no native facilities for attaching binary files. (The org-attach family of commands copy the attachment into a subdirectory of the current directory, and are therefore unrelated to what you are asking.)
Of course, there's nothing preventing you from encoding your binary file as plain text (...
defines URL + destination domain settings
run the <select file> emacs process
exec the file upload using cadaver/lftp/netrc/...
grab/build the destination file-link (may need to use the ownCloud Share API)
paste URL at emacs under cursor
Or alternatively start with a simple alias which, for a given file (or directory) uploads it and ...
[Capturing and summarizing the comments as an answer.]
Although org-attach does not provide an automatic way to add the list of files in the attachment directory of a headline node to that node explicitly, it does provide some mechanisms which could be used to do so. There is no problem about getting the current list:
Maybe you can try auto-answer.
I don't use mu4e, so the following snippet is just a pseudo-code.
(defun my/mu4e-save-all-attachments (fpath)
(let ((auto-answer '(("<prompt-regex>" fpath))
Each prompt from mu4e will be answered by the first prompted fpath.
Just to summarize the exchange in the comments:
org-attach-attach used to add an Attachments property up to, and including Org mode version 9.2.4. Since version 9.2.5 (more specifically, since commit ae9cd437) this is no longer the case. Here are the details of the relevant commit (but note that there is no mention of the specific change that is the subject ...
I briefly Googled, but did not find an example of a multiple file extract function that I thought I had once read somewhere. I only found a multiple attach example. Here is a simplified version of what I use in my own-setup -- I like to check for an existing file with the same name as I extract all files from a single message, and if there is none, then I ...