When downloading message resources, the saved filename was always derived from
file_key (e.g. file_v2_abc123.xlsx), ignoring the original filename the
sender uploaded. This PR resolves filenames from the Content-Disposition
response header first, falling back to Content-Type-based extension inference
only when the header is absent.
Change-Id: I68b48cf428aa8aded4ad9d55fa042f9d68263c3a