MIME Filter Browser Extension

Block what doesn't belong. Allow what does.

A cross-browser extension for Chrome, Microsoft Edge, Brave, Firefox, LibreWolf, and Tor that intercepts browser downloads and blocks or permits them based on user-defined MIME type rules.

MIME Filter is now live on the Chrome Web Store → Get it here. No developer mode, no manual loading, just install and go. If you're already using it, a quick rating on the store page helps other people find it.

Installation

Google Chrome (Chrome Web Store)

1. Visit the Chrome Web Store listing.
2. Click Add to Chrome.
3. The ⬡ icon appears in the toolbar. Done.

Microsoft Edge (from the Chrome Web Store)

1. Visit the Chrome Web Store listing in Edge.
2. Click Add to Chrome. Edge will prompt you to confirm installing an extension from another store, click Allow extension from other stores (or enable it once in edge://extensions if you don't see the prompt).
3. Click Add extension to confirm.
4. The ⬡ icon appears in the toolbar.

Brave (from the Chrome Web Store)

1. Visit the Chrome Web Store listing in Brave.
2. Click Add to Chrome, then Add extension to confirm. Brave installs it straight from the Chrome Web Store, no extra toggle needed.
3. The ⬡ icon appears in the toolbar.

Firefox/LibreWolf/Tor (signed .xpi - permanent install)

1. Download the latest .xpi file from the Releases page.
2. Open Firefox and go to about:addons.
3. Click the gear icon ⚙️ → Install Add-on From File…
4. Select the downloaded .xpi file.
5. Click Add when prompted.

The extension will persist across Firefox restarts and update when you install a newer .xpi.

For LibreWolf and Tor, the steps are identical, the signed .xpi works without any config changes.

Firefox/LibreWolf/Tor (temporary install)

1. Open about:debugging#/runtime/this-firefox.
2. Click Load Temporary Add-on → select manifest.json inside mime-filter/.
3. The extension is active until Firefox restarts.

Usage

Enable/Disable

The ON/OFF toggle in the header enables or disables all download filtering. Settings and logs are preserved while disabled.

Mode

Mode	Behaviour
Allowlist	Only MIME types matching a rule are permitted. Everything else is blocked.
Denylist	MIME types matching a rule are blocked. Everything else passes through.

Adding Rules

1. Type a MIME type or prefix into the input field and press Enter or click ＋.
2. Or click a Quick add tag for common types.

Rules are prefix-matched (case-insensitive):

Rule entered	Matches
application/pdf	Exactly application/pdf
image/	image/png, image/jpeg, image/webp, …
text/	text/plain, text/html, text/csv, …
application/vnd.openxmlformats-officedocument.	All modern Office formats

Log

The Log tab shows all intercepted downloads, newest first. Each entry records:

• Status: blocked or allowed
• MIME type detected by the browser
• Source URL
• Timestamp

Click Export JSON to download the full log as a .json file.

Sample MIME Type Rules

Allowlist (office + documents)

application/pdf
image/
text/plain
text/csv
application/json
application/zip
application/vnd.openxmlformats-officedocument.
application/vnd.oasis.opendocument.
application/pkcs12
audio/
video/

Denylist (block executables)

application/x-msdownload
application/x-executable
application/x-sh
application/x-bat
application/x-msi
application/octet-stream

Sample Log Output (JSON)

[
  {
    "id": "42",
    "url": "https://example.com/report.pdf",
    "filename": "report.pdf",
    "mimeType": "application/pdf",
    "status": "allowed",
    "reason": "MIME type \"application/pdf\" matched allowlist rule",
    "timestamp": "2026-03-11T09:14:22.801Z"
  },
  {
    "id": "43",
    "url": "https://evil.example.com/payload.exe",
    "filename": "payload.exe",
    "mimeType": "application/x-msdownload",
    "status": "blocked",
    "reason": "MIME type \"application/x-msdownload\" is not in the allowlist",
    "timestamp": "2026-03-11T09:15:03.412Z"
  }
]

MIME Matching Flow (background.js)

chrome.downloads.onCreated
        │
        ▼
  filtering enabled? ──No──▶ pass through
        │ Yes
        ▼
  extract downloadItem.mime
        │
        ▼
  matchesMimeRule(mime, rules)
  (prefix-matched, case-insensitive)
        │
   ┌────┴────┐
   │         │
Allowlist  Denylist
   │         │
match=allow  match=block
no match=block  no match=allow
        │
        ▼
  Block: cancel() → erase() → notify() → log(blocked)
  Allow: log(allowed)

Browser Compatibility

Feature	Chrome MV3	Microsoft Edge MV3	Brave MV3	Firefox MV3	LibreWolf/Tor
Download interception	✅	✅	✅	✅	✅
Cancel download	✅	✅	✅	✅	✅
Notifications	✅	✅	✅	✅	✅
Persistent storage	✅	✅	✅	✅	✅
Service worker	✅	✅	✅	✅ (109+)	✅

Note: Firefox requires the browser_specific_settings.gecko.id field in manifest.json; this is already included.

License

Distributed under the MIT License. See LICENSE.

Chrome Web Store poster