Update README.md to explain custom file use case

Add documentation to indicate a use case for custom file values and options in multipart form data requests. 

The added sentence may have saved my team a few hours.

Our specific use case:
- We wanted to receive uploaded files and send them directly to an external service without allowing the files to be written to disk.
- Because we were uploading a file from a stream, we needed to add custom file-related information in our request to the external service.