You can put the files in S3 or NFS like that and send the link/bool to rasa core that perceives that it has received a file.
another option would be to store the byte stream in the tracker itself though I am not sure it is a good idea.
All you need is some sort of indicator that you provide to the bot that you have received a file, unless you want the NLU to parse the file itself for information but that also you should do with a different API, bot should process relevant natural text or slots that alter the direction of the conversation