What I'd like to explore therefore is whether we
could have B's server pull
a copy of the file for B to access, processing and retaining it as B's
organisation requires.
I have some code here which effectively does this:
https://modules.prosody.im/mod_muc_cache_media.html it is written for MUC
but if you aren't doing e2ee the same approach could easily work for
non-MUC.