GitHub Attachment Preview

Displays authenticated GitHub user-attachments images and videos in VS Code's built-in Markdown preview.

GitHub may require authentication for attachment URLs even when the repository is public. Browsers use the signed-in GitHub session, while VS Code's Markdown preview normally requests the attachment anonymously and receives a 404.

Usage

1. Sign in to GitHub from VS Code's Accounts menu.
2. Open a Markdown file containing a GitHub attachment in a trusted workspace.
3. Run Markdown: Open Preview.
4. Approve repository access when VS Code prompts.

The first render starts an authenticated download. The preview refreshes automatically when the attachment is ready.

Supported markup

Markdown images:

![alt](https://github.com/user-attachments/assets/11111111-2222-4333-8444-555555555555)

Standalone attachment URLs are detected as images or videos from their MIME type:

https://github.com/user-attachments/assets/11111111-2222-4333-8444-555555555555

HTML image, video, and source elements:

<img src="https://github.com/user-attachments/assets/11111111-2222-4333-8444-555555555555">

<video controls src="https://github.com/user-attachments/assets/11111111-2222-4333-8444-555555555555"></video>

<video controls>
  <source src="https://github.com/user-attachments/assets/11111111-2222-4333-8444-555555555555">
</video>

Media formats

• Images: PNG, JPEG, GIF, WebP, BMP, and TIFF signatures, plus formats such as SVG and AVIF when GitHub returns a valid image/* MIME type.
• Videos: MP4, WebM, and MOV.

Rendering ultimately depends on the image decoder and video codecs available in VS Code's embedded browser. In particular, MOV playback varies by codec.

Scope and limits

• Only exact HTTPS github.com/user-attachments/assets/<id> URLs are handled.
• The extension stays disabled in Restricted Mode and never downloads media from an untrusted workspace.
• Ordinary remote images and repository file links are unchanged.
• PDFs, archives, logs, audio, and other unsupported attachments remain links.
• Images default to a 20 MB limit and videos to 100 MB. Both are configurable.
• Images are held in extension memory. Videos are streamed to VS Code's extension storage directory to avoid Base64 memory amplification.
• Cached media is released when the cache is cleared, the active GitHub session changes, or VS Code closes.
• GitHub credentials never enter Markdown, generated HTML, files, logs, or the preview webview.

Build and install

npm install
npm run package
code --install-extension github-attachment-preview-*.vsix

Settings

• githubAttachmentPreview.maxImageSizeMB: maximum image size loaded into the preview. Defaults to 20 MB.
• githubAttachmentPreview.maxVideoSizeMB: maximum video size loaded into the preview. Defaults to 100 MB.

Use GitHub Attachment Preview: Clear Attachment Cache to clear downloaded attachments and retry failed requests.