Allow changing attachment file name when saving it

[Tristan02d]

Hello, I think there is currently no way to change an attachment file name when saving it. Say
there is an attachment called not Convenient file Name.PDF, I would like to save it as
convenient_file_name.pdf.

Here is my attempt to make it possible, using the new mu4e-view-save-one-attachment
command, tell me what you think.

[djcb]

Thanks for your PR.

Note that you can change the name of file when saving the attachment when going through mu4e-view-mime-part-action, i.e., "A", then select the attachment and "s".

But, perhaps we can improve the direct way as well; however, I would not start with a function mirroring the existing mu4e-save-attachments. People may want any combination of custom-dir / custom-name. So perhaps a new combination function that handles all combinations of ask-dir / ask-filename (i.e., a superset of mu4e-save-attachments, and then perhaps some helpers using it.

WDYT?

[Tristan02d]

mu4e-view-mime-part-action does exactly what I wanted, thus
mu4e-view-save-one-attachment is kind of pointless now. Thank you for pointing
that out.

Making all the custom-dir / custom-name combination conveniently reachable from the
mu4e-view-mode would indeed be super cool.

The question is how to make them accessible ? I would say the easiest way would
be to add the 4 variants of the "save" action to
mu4e-view-mime-part-action. But I think being able to bind the one we use the
most to a single keystroke in mu4e-view-mode would be very useful, but that
implies providing one command for each of the 4 variant.

[djcb]

Yeah, I'd expect to have one function to handle the variations, both with custom dir / custom filename, and let user choose influence through prefix-args, and perhaps some user-helpers on top. I'd like to avoid adding too many key-bindings. Have to think a bit about how to best present this to the user.