Content-Disposition filename parameter should not include directories

[FrancisHe]

RFC 1806:

The receiving MUA should not respect any directory path information
that may seem to be present in the filename parameter. The filename
should be treated as a terminal component only. Portable
specification of directory paths might possibly be done in the future
via a separate Content-Disposition parameter, but no provision is
made for it in this draft.

That means:
if the local file is "test/test.txt", then the the "filename" parameter should be "test.txt", not "test/test.txt".