Allow specifying file naming format

[JuniorJPDJ]

eg. providing parameter like --format '{msgid}_{filename}'

[nktknshn]

How are you going to use this feature? Would you please give me some examples of custom file naming.

[jpruciak]

Legacy tgmount has {msgid} {filename}.{fileext} format for normal files.
Example: 77876 Sonata Arctica - The Days of Grays (2009).zip.
And msg_{msgid}_doc for unnamed files.
Example: msg_221285_doc.

I'd like to be able to configure the naming and for example make it {filename}.{msgid}.{fileext} for named and {msgid}.{fileext} for unnamed. fileext can be guessed from mimetype if filename is not present.
This will for example allow easily unpacking multipart archives.

Or just for {msgid}_{filename}.{fileext} so there's no space between msgid and filename.

Or for people not needing msgid at all they could set it to {filename}.{fileext} and that would be cool for them.