You're implementing an API endpoint that should return a file to the user. However, if the file is not found, you want to return a custom error message in JSON format. How can you best achieve this mixed response type?

To achieve the desired behavior of returning a file if found or a custom error message in JSON format if not found, you can use the File action result. This result allows you to return a file if it exists or handle the "file not found" scenario by constructing a custom JSON response. It provides the flexibility needed for this mixed response type.