feat: support attachments in ChatGPT browser conversations

[lalalic]

Summary

Extend chatgpt_conversation_start so callers can send HTTPS-backed files through the signed-in ChatGPT browser runtime and receive persisted assistant-generated files/images as structured outputs.

Input

chatgpt_conversation_start accepts up to four attachments with url, name, optional mime_type, and optional declared size. The Chrome extension fetches the HTTPS source, stages bounded bytes in memory, mounts them through ChatGPT's native composer file input, and submits through the existing first-party submitComposer runtime path.

Output

After completion, persisted assistant content is inspected for downloadable files/images. Returned artifacts are exposed as assistant_outputs entries containing type, name, MIME type, size, and base64 bytes. ChatGPT document-card downloads are resolved through the signed-in page without exposing browser credentials.

Safety / bounds

• HTTPS source URLs only
• maximum 4 inputs
• bounded per-file and aggregate staging sizes
• output byte bounds
• existing ChatGPT credential/proof-field rejection remains unchanged
• no API keys or browser credentials leave ChatGPT

Validation

• npm run check
• node tests/chatgpt-runtime.mjs
• node tests/chrome-background.mjs
• live signed-in browser validation with an image attachment and a generated text-file output

The feature is transport- and application-agnostic; it is not tied to any specific caller or file host.