For file uploads, I’ve got this working with a very small example (still need to test this for larger files/binaries):
https://community.openai.com/t/unable-to-upload-files-from-a-custom-chatgpt-session-via-an-api-action
TL;DR - Have it break the base64 encoded file into 500 character chunks and send those chunks to an endpoint that will reassemble them into the file.