Take an optional index parameter for a *subpart*.
This is for the case where we save attachments from a message (in particular,
when forwarding). We can't save them in the same directory for the (rare) case
when there are multiple attachments with the same name. And we don't want to
uniquify the name, since that shows up in e.g. the forwarded file name.
This can be solved by saving each in their own indexed subdir.
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