fix: Support local file storage uploads in addition to S3

The upload endpoint now detects whether Outline uses local file storage
(multipart form POST) or S3 (pre-signed PUT URL) and handles both.

server.py

@@ -633,27 +633,50 @@ async def upload_endpoint(request: Request) -> JSONResponse:
             status_code=502,
         )
 
-    attachment_data = create_response.get("data", {})
-    upload_url = attachment_data.get("uploadUrl")
+    response_data = create_response.get("data", {})
+    upload_url = response_data.get("uploadUrl")
+    form_fields = response_data.get("form", {})
+    attachment_data = response_data.get("attachment", {})
     if not upload_url:
         return JSONResponse(
             {"ok": False, "error": "Outline API did not return an uploadUrl"},
             status_code=502,
         )
 
-    # 3. Stream the request body directly to S3 upload_url
+    # Resolve relative uploadUrl against the Outline base URL
+    if upload_url.startswith("/"):
+        upload_url = f"{OUTLINE_API_URL}{upload_url}"
+
+    is_local_storage = bool(form_fields)
+
+    # 3. Upload the file content to storage
     try:
-        # Pre-signed S3 PUT URLs usually enforce exact headers
+        body = await request.body()
+
+        if is_local_storage:
+            # Local/database file storage: POST multipart form with fields from Outline
+            logger.info(f"Uploading via local storage endpoint (size={size} bytes)")
+            files = {"file": (filename, body, content_type)}
+            async with httpx.AsyncClient() as client:
+                post_response = await client.post(
+                    upload_url,
+                    data=form_fields,
+                    files=files,
+                    headers={"Authorization": f"Bearer {OUTLINE_API_TOKEN}"},
+                    timeout=600.0,
+                )
+                post_response.raise_for_status()
+        else:
+            # S3-compatible storage: PUT directly to pre-signed URL
             headers = {
                 "Content-Type": content_type,
                 "Content-Length": str(size),
             }
-
-        logger.info(f"Streaming file content to storage (size={size} bytes)")
+            logger.info(f"Streaming file content to S3 storage (size={size} bytes)")
             async with httpx.AsyncClient() as client:
                 put_response = await client.put(
                     upload_url,
-                content=request.stream(),
+                    content=body,
                     headers=headers,
                     timeout=600.0,
                 )