You can use the EmailConnector module. This module can download messages from your inbox.
After retrieving the messages you can build a microflow which can store the data in any place you want in Mendix.
I think the easiest way is to using a Published REST API in mendix.
Check this documentation: https://docs.mendix.com/refguide/send-receive-files-rest/
So from the external client(the caller) you send a file to the mendix exposed REST API which has a microflow underneath it to parse the file and perhaps parse it(for e.g.: if the file was excel) or if its an image, store it for display on other pages.