docs: add external document extraction documentation

docs/features/chat-conversations/rag/document-extraction/external.md

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+---
+sidebar_position: 5
+title: "External Document Extraction"
+---
+
+# External Document Extraction
+
+The **External** extraction method allows you to delegate the document extraction process entirely to your own custom API or an external HTTP microservice. 
+
+This is the most flexible and scalable option, as it allows you to:
+- Integrate proprietary OCR or extraction tools.
+- Use cloud-based third-party extraction APIs.
+- Offload heavy extraction tasks from the main Open WebUI backend to a dedicated server with GPUs or more memory.
+- Filter or redact PII (Personally Identifiable Information) before the text is stored in Open WebUI.
+
+## How it Works
+
+When the **Content Extraction Engine** is set to **External** in the Open WebUI Admin Settings, Open WebUI will bypass its internal parsing libraries. Instead, it sends the raw, uploaded file to your specified external service via an HTTP `PUT` request and expects the extracted text to be returned in a specific JSON format.
+
+## Configuration
+
+To use this method, navigate to **Admin Panel** -> **Settings** -> **Documents** -> **Content Extraction Engine** and select **External**.
+
+You will then need to provide the following settings:
+
+- **URL**: The base URL of your external extraction service. Open WebUI will automatically append `/process` to this URL. *(Note: Ensure your URL does not have a trailing slash, or it will become `{URL}//process`).*
+- **API Key (Optional)**: If your external service requires authentication, you can provide an API Key here. Open WebUI will pass it as a Bearer token.
+
+## API Specification (For Developers)
+
+If you are building the external service to handle the extraction, it must conform to the following API specification.
+
+### Request
+Open WebUI will send a request to `{YOUR_BASE_URL}/process` with the following properties:
+
+- **Method**: `PUT`
+- **Body**: The raw binary content of the uploaded file.
+- **Headers**:
+  - `Content-Type`: The MIME type of the uploaded file (e.g., `application/pdf`, `image/jpeg`, `text/plain`).
+  - `Authorization`: `Bearer {API_KEY}` (Only included if an API Key is configured in Open WebUI).
+  - `X-Filename`: The URL-encoded original filename of the uploaded document.
+  - *Additional Headers*: Open WebUI may also forward custom headers or user-specific metadata headers depending on your environment variables.
+
+### Expected Response
+Your service must return an HTTP `200 OK` status with a `JSON` payload. The payload can be either a single JSON object or a JSON array of objects.
+
+Each object **must** contain the following keys:
+- `page_content` (string): The extracted text.
+- `metadata` (object): A dictionary containing metadata about the extracted content (e.g., page number, source filename).
+
+#### Example JSON Response
+```json
+[
+    {
+        "page_content": "This is the extracted text from the first page of the document.",
+        "metadata": {
+            "page": 1,
+            "author": "John Doe"
+        }
+    },
+    {
+        "page_content": "This is the extracted text from the second page.",
+        "metadata": {
+            "page": 2,
+            "author": "John Doe"
+        }
+    }
+]
+```
+
+## Example: Building a FastAPI External Extraction Service
+
+Here is a minimal example of a FastAPI server that you can run locally to act as your External Document Extraction Engine. 
+
+```python
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
+import uvicorn
+
+app = FastAPI()
+
+
+@app.api_route("/process", methods=["PUT", "POST"])
+async def external_process(request: Request):
+    original_filename = request.headers.get("x-filename", "")
+    content_type = request.headers.get("content-type", "")
+
+    # Read the raw file bytes
+    file_bytes = await request.body()
+
+    # TODO: Implement your extraction logic here
+    # extracted_text = your_processing_function(file_bytes, content_type)
+    extracted_text = "Placeholder: Replace this with your actual extraction logic."
+
+    return JSONResponse(
+        {
+            "page_content": extracted_text,
+            "metadata": {
+                "source": original_filename or "external_document",
+                "content_type": content_type
+            }
+        }
+    )
+
+
+if __name__ == "__main__":
+    # To run this server:
+    # pip install fastapi uvicorn
+    # python server.py
+    uvicorn.run(app, host="0.0.0.0", port=5000)
+```
+
+Once this server is running on `http://localhost:5000`, set your Open WebUI External URL to `http://localhost:5000`, and Open WebUI will send uploaded files to this script for extraction.