The Content-Disposition header was missing the required disposition-type
(attachment;) and proper encoding for non-ASCII filenames.
Changes:
- Add `create_content_disposition_header()` helper function
- Update all 4 Content-Disposition headers in view_image endpoint
- Provide ASCII fallback filename for legacy clients
- Add RFC 5987 UTF-8 encoded filename* parameter for international support
This fixes third-party download libraries (e.g., Go's mime.ParseMediaType)
failing to parse the filename correctly.
Fixes#8914
