post https://core.nextmatter.com/api/files/
Create a new file for the organization and (optionally) a specific workflow or instance.
Use the URI from the response to reference the file in your workflow.
Note that base64 is not natively supported as a direct upload format.
If you have a base64 file, convert it to binary before uploading.
It accepts a file field in the request data, with a file.