fix(files): include charset in text/* and image/svg+xml downloads

[brianbckim]

Context
Some download responses returned Content-Type without charset while tests expect ; charset=utf-8.

Changes

• Ensure Content-Type includes ; charset=utf-8 for text/* and image/svg+xml
• LOCAL path: Response(..., content_type=...) + explicit header
• Redirect (raw): add response-content-type with charset

Verification
python -m pytest -k "test_web_download or test_api_download" -q