I'm able to create a microflow with a FileDocument as a parameter (lets call this TransferFileDocument) and publish this as an app service in an application wich we will call ASPublisher.
I assume i can now also consume this microflow in a different Mendix application, using app services, lets call this app ASConsumer.
So now everytime ("on demand") when i want to transfer a FileDocument from the ASConsumer app to the ASPublisher app i can just call the TransferFileDocument and the file will be transfered to the other app. You just need to make sure you take care of storing it.
If you want my two test projects, let me know.