fix: resolve empty attachment MIME types from filename

[pranav-afk]

Description

Fixes attachment uploads failing with "Invalid file type" when the browser sends an empty MIME type for text-like files (.md, .csv, .txt), common on macOS.

Allowlisted extensions were already supported, but the API required a non-empty type and rejected type: "". The client also often left type empty after signature detection (text files have no reliable magic bytes).

Changes:

• Resolve content type from filename when the client omits it (mimetypes + extension fallbacks)
• Wire the helper into issue attachment v2, public issue attachment, and generic asset upload endpoints
• Allow text/csv and text/tab-separated-values in ATTACHMENT_MIME_TYPES
• Frontend upload helper: fall back to file.type, then extension map
• Unit tests for the MIME resolution helper

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Improvement (change that would cause existing functionality to not work as expected)
• Code refactoring
• Performance improvements
• Documentation update

Test Scenarios

• Attach .md, .csv, and .txt files to a work item (especially from macOS) — expect success, not 400 Invalid file type
• Attach a normal image (e.g. PNG) — still works
• Attach a disallowed extension — still rejected
• POST attachment metadata with "type": "" and a valid name — server guesses type and accepts if allowlisted
• Run unit tests for resolve_attachment_content_type

References

• Fixes [bug]: Attachments fail with "Invalid file type" when browser sends empty MIME (.md/.csv/.txt on macOS) #9026

Summary by CodeRabbit

• New Features

  • Improved attachment type detection using the file name when MIME information is missing or incomplete.
  • Added support for CSV and tab-separated values attachments.
  • Improved browser-side detection for common text-based files.

• Bug Fixes

  • Reduced attachment upload failures caused by missing or incorrect MIME types.
  • Preserved explicitly provided content types after trimming whitespace.

[coderabbitai]

📝 Walkthrough

Walkthrough

Attachment handling now resolves missing or empty MIME types from filenames across backend upload endpoints and frontend file detection, with added support for CSV and tab-separated values plus backend unit coverage.

Changes

Attachment MIME resolution

Layer / File(s)	Summary
MIME resolver and supported types apps/api/plane/utils/attachment.py, apps/api/plane/settings/common.py, apps/api/plane/tests/unit/utils/test_attachment.py	Adds filename-based MIME resolution, expands allowed text MIME types, and tests explicit-type normalization and filename fallbacks.
Backend upload integration apps/api/plane/api/views/asset.py, apps/api/plane/api/views/issue.py, apps/api/plane/app/views/issue/attachment.py	Uses the resolver before attachment validation and asset creation across upload endpoints.
Frontend file detection packages/services/src/file/helper.ts	Uses browser-provided MIME types or known filename extensions when signature detection does not identify a type.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• makeplane/plane#8524: Updates attachment MIME allowlisting for Markdown, which this change extends with filename-based resolution.

Suggested reviewers: dheeru0198, pablohashescobar, sriramveeraghanta

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: resolving empty attachment MIME types from filenames.
Description check	✅ Passed	The description includes the required sections and covers the change, testing, type of change, and references.
Linked Issues check	✅ Passed	The changes satisfy #9026 by inferring MIME types from filenames, adding client-side fallback, and preserving allowlist validation.
Out of Scope Changes check	✅ Passed	No clearly unrelated changes stand out; the added helper, tests, allowlist updates, and client fallback all support the attachment MIME fix.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

apps/api/plane/settings/common.py (1)

476-477: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Duplicate "text/csv" entry in ATTACHMENT_MIME_TYPES.

"text/csv" is now present twice in the ATTACHMENT_MIME_TYPES list (it is already listed further down in the file). Consider removing the duplicate.

♻️ Proposed refactor

     "text/markdown",
-    "text/csv",
     "text/tab-separated-values",
     "application/rtf",

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@apps/api/plane/settings/common.py` around lines 476 - 477, Remove the newly
added duplicate "text/csv" entry from ATTACHMENT_MIME_TYPES, keeping the
existing occurrence elsewhere in the list and retaining
"text/tab-separated-values".

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@apps/api/plane/settings/common.py`:
- Around line 476-477: Remove the newly added duplicate "text/csv" entry from
ATTACHMENT_MIME_TYPES, keeping the existing occurrence elsewhere in the list and
retaining "text/tab-separated-values".

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a83c9704-a1a1-4a18-ab49-34c24a93bb3a

📥 Commits

Reviewing files that changed from the base of the PR and between bed58d9 and 90c877e.

📒 Files selected for processing (7)

• apps/api/plane/api/views/asset.py
• apps/api/plane/api/views/issue.py
• apps/api/plane/app/views/issue/attachment.py
• apps/api/plane/settings/common.py
• apps/api/plane/tests/unit/utils/test_attachment.py
• apps/api/plane/utils/attachment.py
• packages/services/src/file/helper.ts