fix(upload): send browser content type for direct uploads

[mumingluan]

Summary / 摘要

Send the browser's File.type in the optional content_type field when requesting a direct-upload URL. Use application/octet-stream when the browser provides no type.

This lets the companion backend change sign the client-selected Content-Type and return the matching upload header, fixing Ceph RGW uploads that reject an unsigned media type. The upload code already applies the headers returned by the backend.

• This PR has breaking changes.
• This PR changes public API, config, storage format, or migration behavior.
  Adds the optional content_type field to the direct-upload information request.
• This PR requires corresponding changes in related repositories.

Related repository PRs / 关联仓库 PR:

• OpenList: fix(drivers/s3): sign content-type for direct uploads OpenList#3152

Testing / 测试

• Prettier check for src/pages/home/uploads/direct.ts and git diff --check passed.
• Executed the changed upload function in an isolated Node File/XHR harness. A .apk with text/plain and a file with an empty type send the expected request field and matching upload header.
• With the companion backend driver, Rainyun/Ceph and Cloudflare R2 uploads succeeded when the supplied MIME type differed from the extension. An 80 MiB Rainyun upload using the empty-type fallback also succeeded. Object sizes were verified and temporary objects were deleted.
• pnpm run lint reports seven errors in unrelated files. TypeScript checking at the original frontend HEAD produced identical diagnostics.

Checklist / 检查清单

• I have read the linked OpenList contribution instructions and the PR template.
• The contribution follows the repository license and contribution format.
• The changed code is formatted with Prettier.
• The human collaborator reviewed the proposed change and approved submission.

AI Disclosure / AI 使用声明

• This PR includes AI-assisted content.
• Tools used: Codex.
• Usage scope: implementation, validation, and PR wording.
• Codex performed the checks described above. The human collaborator requested the change in response to maintainer feedback, reviewed it, and approved submission.
• AI-assisted commits include the required Co-authored-by attribution.