Include RENDER_EXTERNAL_URL in .env.example

[ben-kenney]

When using powerpoint, excel or word plugins that require downloading files from the plugins server, the default behaviour is to reference the server as http://localhost:3000 which will fail when the user tries to download the files.

For example, here's the output after using the powerpoint plugin on TypingMind:

{"success":true,"message":"File generated successfully","responseObject":{"downloadUrl":"http://localhost:3000/powerpoint-generator/downloads/your-presentation-20250313095508748.pptx"},"statusCode":200}

To fix this users can add the RENDER_EXTERNAL_URL variable to their .env. I recommend the .env.example be updated with the following line to make it more obvious how to fix this issue:

RENDER_EXTERNAL_URL="http://localhost:3000" # replace with your plugins server endpoint url