[bug] Web UI: Chinese Markdown files show garbled/mojibake text when opened via /api/files/:id/content (Content-Type missing charset)

[zlx]

Description

After uploading a Markdown file containing Chinese text, opening it (via /api/files/{id}/content) shows the Chinese characters as garbled mojibake.

Steps to Reproduce

1. Sign in to the QM Web UI (dev mode)
2. Upload a Markdown file containing Chinese text, e.g. content # 标题\n这是中文测试
3. Click the file to open /api/files/{id}/content in the browser
4. Observe that the Chinese characters render as mojibake (e.g. æ ‡é¢˜)

Expected Behavior

Chinese content should render correctly as UTF-8 text.

Actual Behavior

Chinese text is displayed as garbled mojibake.

Environment

• QM version: main branch (commit 0f0e0ad)
• Deployment: local, Web UI dev (cookie-auth) mode
• Browser: Chrome
• Node: v24

Notes
• The filename is already correctly UTF-8 encoded via filename*=UTF-8''..., confirming the file itself is stored as UTF-8
• The issue is the missing charset declaration in the response Content-Type header