Depends a bit on the server that stores thoses files. You have multiple options like SFTP or use a webservice to transfer the files. Do note that that server already needs to have the account credentials of that user unless that server has some means to create those with for instance a webservice.
Is there a specific reason why those files need to be on that external server? Otherwise I would just do all in Mendix. Much easier to create and more secure.
We worked with many vendors and the common request from them are they need some log file from our server whenever they are troubleshooting issue. Due to the number of vendor we have, this has consumed many of our support team time and because the request came in at random time, it break up the time into several small piece, in the long run also not good for the team productivity.
About the suggestions you made, do you means that it is possible to create an apps like what I describe with Mendix using SFTP/webservice?
Thanks for the suggestion, I shall explore more on Mendix platform to understand how to do that.