feat: add attachment download via proxy endpoint

Add GET /api/v1/attachments/{id} endpoint for downloading attachments
through the MCP proxy. This complements the existing upload endpoint and
enables complete attachment workflows via the proxy API.

src/grist_mcp/grist_client.py

@@ -149,6 +149,38 @@ class GristClient:
                 "size_bytes": len(content),
             }
 
+    async def download_attachment(self, attachment_id: int) -> dict:
+        """Download an attachment by ID.
+
+        Args:
+            attachment_id: The ID of the attachment to download.
+
+        Returns:
+            Dict with content (bytes), content_type, and filename.
+        """
+        import re
+
+        async with httpx.AsyncClient(timeout=self._timeout) as client:
+            response = await client.get(
+                f"{self._base_url}/attachments/{attachment_id}/download",
+                headers=self._headers,
+            )
+            response.raise_for_status()
+
+            # Extract filename from Content-Disposition header
+            content_disp = response.headers.get("content-disposition", "")
+            filename = None
+            if "filename=" in content_disp:
+                match = re.search(r'filename="?([^";]+)"?', content_disp)
+                if match:
+                    filename = match.group(1)
+
+            return {
+                "content": response.content,
+                "content_type": response.headers.get("content-type", "application/octet-stream"),
+                "filename": filename,
+            }
+
     # Schema operations
 
     async def create_table(self, table_id: str, columns: list[dict]) -> str:

src/grist_mcp/main.py

@@ -363,6 +363,67 @@ def create_app(config: Config):
                 "code": "GRIST_ERROR",
             })
 
+    async def handle_attachment_download(
+        scope: Scope, receive: Receive, send: Send, attachment_id: int
+    ) -> None:
+        """Handle attachment download by ID."""
+        # Extract token
+        token = _get_bearer_token(scope)
+        if not token:
+            await send_json_response(send, 401, {
+                "success": False,
+                "error": "Missing Authorization header",
+                "code": "INVALID_TOKEN",
+            })
+            return
+
+        # Validate session token
+        session = token_manager.validate_token(token)
+        if session is None:
+            await send_json_response(send, 401, {
+                "success": False,
+                "error": "Invalid or expired token",
+                "code": "TOKEN_EXPIRED",
+            })
+            return
+
+        # Check read permission
+        if "read" not in session.permissions:
+            await send_json_response(send, 403, {
+                "success": False,
+                "error": "Read permission required for attachment download",
+                "code": "UNAUTHORIZED",
+            })
+            return
+
+        # Download from Grist
+        try:
+            doc = auth.get_document(session.document)
+            client = GristClient(doc)
+            result = await client.download_attachment(attachment_id)
+
+            # Build response headers
+            headers = [[b"content-type", result["content_type"].encode()]]
+            if result["filename"]:
+                disposition = f'attachment; filename="{result["filename"]}"'
+                headers.append([b"content-disposition", disposition.encode()])
+
+            await send({
+                "type": "http.response.start",
+                "status": 200,
+                "headers": headers,
+            })
+            await send({
+                "type": "http.response.body",
+                "body": result["content"],
+            })
+        except Exception as e:
+            await send_json_response(send, 500, {
+                "success": False,
+                "error": str(e),
+                "code": "GRIST_ERROR",
+            })
+
     async def app(scope: Scope, receive: Receive, send: Send) -> None:
         if scope["type"] != "http":
             return
@@ -380,6 +441,17 @@ def create_app(config: Config):
             await handle_proxy(scope, receive, send)
         elif path == "/api/v1/attachments" and method == "POST":
             await handle_attachments(scope, receive, send)
+        elif path.startswith("/api/v1/attachments/") and method == "GET":
+            # Parse attachment ID from path: /api/v1/attachments/{id}
+            try:
+                attachment_id = int(path.split("/")[-1])
+                await handle_attachment_download(scope, receive, send, attachment_id)
+            except ValueError:
+                await send_json_response(send, 400, {
+                    "success": False,
+                    "error": "Invalid attachment ID",
+                    "code": "INVALID_REQUEST",
+                })
         else:
             await handle_not_found(scope, receive, send)
 

src/grist_mcp/tools/session.py

@@ -8,9 +8,10 @@ PROXY_DOCUMENTATION = {
     "description": "HTTP proxy API for bulk data operations. Use request_session_token to get a short-lived token, then call the proxy endpoint directly from scripts.",
     "endpoints": {
         "proxy": "POST /api/v1/proxy - JSON operations (CRUD, schema)",
-        "attachments": "POST /api/v1/attachments - File uploads (multipart/form-data)",
+        "attachments_upload": "POST /api/v1/attachments - File uploads (multipart/form-data)",
+        "attachments_download": "GET /api/v1/attachments/{id} - File downloads (binary response)",
     },
-    "endpoint_note": "The full URL is returned in the 'proxy_url' field of request_session_token response. Replace /proxy with /attachments for file uploads.",
+    "endpoint_note": "The full URL is returned in the 'proxy_url' field of request_session_token response. Replace /proxy with /attachments for file operations.",
     "authentication": "Bearer token in Authorization header",
     "attachment_upload": {
         "endpoint": "POST /api/v1/attachments",
@@ -27,6 +28,20 @@ response = requests.post(
 )
 attachment_id = response.json()['data']['attachment_id']
 # Link to record: update_records with {'Attachment': [attachment_id]}""",
+    },
+    "attachment_download": {
+        "endpoint": "GET /api/v1/attachments/{attachment_id}",
+        "permission": "read",
+        "description": "Download attachment by ID. Returns binary content with appropriate Content-Type and Content-Disposition headers.",
+        "response_headers": ["Content-Type", "Content-Disposition"],
+        "example_curl": "curl -H 'Authorization: Bearer TOKEN' URL/api/v1/attachments/42 -o file.pdf",
+        "example_python": """import requests
+response = requests.get(
+    f'{base_url}/api/v1/attachments/42',
+    headers={'Authorization': f'Bearer {token}'}
+)
+with open('downloaded.pdf', 'wb') as f:
+    f.write(response.content)""",
     },
     "request_format": {
         "method": "Operation name (required)",