fix(drivers/s3): sign content-type for direct uploads

[mumingluan]

Summary / 摘要

S3 browser uploads can fail with 403 AccessDenied on Ceph RGW when the request carries an unsigned Content-Type. The frontend sends file.type || "application/octet-stream" in a new optional content_type field on the direct-upload information request.

The backend validates the supplied media type, passes it to the driver through a typed context key, signs it in the S3 PutObject URL, and returns the same value in the existing upload headers. Missing types fall back to application/octet-stream, preserving compatibility with existing clients.

• This PR has breaking changes.
• This PR changes public API, config, storage format, or migration behavior.
  Adds an optional request field and populates the existing optional response headers.
• This PR requires corresponding changes in related repositories.
  A companion frontend change supplies the browser's MIME type.

Related repository PRs / 关联仓库 PR:

• OpenList-Frontend: fix(upload): send browser content type for direct uploads OpenList-Frontend#692

Testing / 测试

• go test ./drivers/s3 ./internal/conf -count=1 passed.
• The handles package compiles; its full test run fails in TestIsSearchNodeAccessible/outside_base_path. The same failure was reproduced at the original branch HEAD.
• Frontend Prettier and both repositories' git diff --check passed.
• Frontend TypeScript checking reports seven errors in unrelated files. Running it at the original frontend HEAD produced identical diagnostics.
• Executed the revised frontend upload function with an isolated Node File/XHR harness: a .apk carrying text/plain and a file with an empty type send the expected content_type and matching upload header.
• Ran the revised S3 driver against Rainyun and Cloudflare R2. A .apk using the client-supplied text/plain and a .jpg using application/pdf uploaded successfully. Missing MIME type used application/octet-stream, and an 80 MiB Rainyun upload succeeded. Object sizes were verified and temporary objects were deleted.

Checklist / 检查清单

• Read the contribution instructions and PR templates.
• Formatted the changed Go and TypeScript code.
• Human review of this revision is complete.

AI Disclosure / AI 使用声明

• This revision includes AI-assisted content.

• Tool: Codex.

• Scope: implementation, validation, and PR wording.

• The human collaborator requested this revision in response to maintainer feedback. Codex performed the checks described above; the human collaborator reviewed the proposed changes and approved submission.

• AI-assisted commits include the required Co-authored-by attribution.

[xrgzs]

@mumingluan Have you tested other S3 backends?

[mumingluan]

@mumingluan Have you tested other S3 backends?

Only Ceph encountered this issue.
Other backends can work properly with and without the fix.

[xrgzs]

I don't think getting Content-Type from the filename is a good idea. The browser's implementation may differ from the Go backend.

[mumingluan]

Thanks for the feedback. Updated this PR to use the browser-provided MIME type, with the companion frontend change in OpenListTeam/OpenList-Frontend#692.

The frontend sends file.type || "application/octet-stream" as content_type. The backend validates and signs that value, then returns the matching upload header. Existing clients that omit the field use application/octet-stream.

I verified the revised driver against Rainyun/Ceph and Cloudflare R2 with supplied MIME types that differ from the filename extensions, including a successful 80 MiB Rainyun upload using the fallback. Object sizes were verified and the temporary objects were deleted.