Hi,

As Odoo is full API framework, all the action you do in the Ui could be done in API. Odoo UI is a Client that use xmlrpc to send messages/actions to Odoo server.

So the answer is yes. To upload file you'll have first to encore in base64 your bianry so that the XMLRPC protocol accept the text value as a binary field. It's mostly the way emails are working.

Regarding CMIS protocol, some OCA repo https://github.com/OCA/connector-cmis are dealing with this.

regards