This project is an example of a File REST API that shows how to POST a file. It includes the RAML spec and the Mule application. The Mule app, takes the file as multipart/form-data and and saves it to a folder in the app. This can be re-purposed to save the file to S3 or an FTP server.
When making the POST request, set the body to form-data and make sure the key for the file is named fileName