feat: Add embedMarkdown to upload response and document attachment format

The upload response now includes an embedMarkdown field with the correct
Outline attachment syntax ([name size](/api/attachments.redirect?id=...))
so callers can insert it directly into documents for native card rendering.

IMPLEMENTATION.md updated with storage backend details, auth header,
and embedding format documentation.

IMPLEMENTATION.md

@@ -314,6 +314,61 @@ Delete attachment. Params: id (required)
 ### attachments.redirect
 Get attachment URL. Params: id (required)
 
+### Custom HTTP Gateway Upload
+Instead of manually calling `attachments.create` and executing the subsequent storage upload, you can use the MCP server's integrated HTTP upload proxy.
+
+This endpoint receives binary content over HTTP, registers the attachment with Outline, and uploads it to the configured storage backend (S3 or local file storage).
+
+- **Endpoint**: `POST /upload`
+- **Query Parameters**:
+  - `documentId` (required): The UUID of the destination Outline document.
+  - `name` (required): The filename of the attachment (e.g., `image.png`, `document.pdf`).
+- **Headers**:
+  - `Authorization` (required): `Bearer <OUTLINE_API_TOKEN>`
+  - `Content-Length` (required): The exact size of the file in bytes.
+  - `Content-Type` (optional): The MIME type of the file (e.g., `application/pdf`, `image/png`). If omitted, it will be auto-detected by filename extension.
+- **Request Body**: The raw file binary content.
+- **Storage backends**: Auto-detected from the Outline API response.
+  - **S3**: PUT to pre-signed URL.
+  - **Local file storage**: Multipart POST to `/api/files.create` with form fields.
+- **Response**:
+  ```json
+  {
+    "ok": true,
+    "data": {
+      "id": "attachment-uuid",
+      "name": "filename.ext",
+      "size": 12345,
+      "contentType": "mime/type",
+      "url": "/api/attachments.redirect?id=attachment-uuid",
+      "embedMarkdown": "[filename.ext 12345](/api/attachments.redirect?id=attachment-uuid)"
+    }
+  }
+  ```
+
+### Embedding Attachments in Documents
+
+Outline uses a special markdown syntax to render attachment cards (with file icon, name, and download size). The format is:
+
+```
+[filename.ext filesize](/api/attachments.redirect?id=attachment-uuid)
+```
+
+Where:
+- `filename.ext` is the attachment filename
+- `filesize` is the file size in bytes (space-separated, not a label)
+- The URL is the **relative** `/api/attachments.redirect?id=...` path
+
+The `embedMarkdown` field in the upload response provides this string ready to insert into a document body via `documents.update`.
+
+**Example**: Upload a file and embed it in a document:
+```
+1. POST /upload?documentId=doc-uuid&name=report.pdf  →  get embedMarkdown
+2. outline_api_call("documents.update", {"id": "doc-uuid", "text": "...\n\n" + embedMarkdown, "append": true})
+```
+
+**Important**: Do NOT use absolute URLs (e.g., `https://docs.example.com/api/attachments.redirect?id=...`) — Outline will not render the attachment card for absolute URLs. Always use the relative path.
+
 ---
 
 ## Auth

server.py

@@ -684,16 +684,25 @@ async def upload_endpoint(request: Request) -> JSONResponse:
 
         logger.info("Upload to storage completed successfully")
 
-        # 4. Return success and attachment metadata
+        # 4. Return success with attachment metadata and embedding markdown
+        att_name = attachment_data.get("name", filename)
+        att_size = attachment_data.get("size", size)
+        att_url = attachment_data.get("url", "")
+
+        # Outline renders attachment cards when the link text is "filename size"
+        # and the href is the relative /api/attachments.redirect path.
+        embed_markdown = f"[{att_name} {att_size}]({att_url})"
+
         return JSONResponse(
             {
                 "ok": True,
                 "data": {
                     "id": attachment_data.get("id"),
-                    "name": attachment_data.get("name"),
+                    "name": att_name,
-                    "size": attachment_data.get("size"),
+                    "size": att_size,
                     "contentType": attachment_data.get("contentType"),
-                    "url": attachment_data.get("url"),
+                    "url": att_url,
+                    "embedMarkdown": embed_markdown,
                 },
             }
         )