Currently it's not possible to set any validation feedback in a microflow on the file uploader. This means that a message to the user that a file needs to be uploaded is either presented in a separate pop-up message or all kinds of workarounds/hacks are needed to give this feedback.
It would be really nice if it would just be possible to set validation feedback on the file uploader, e.g. by setting it on the "hasContents" attribute of the filedocument.
And as a bonus: an option for onchange microflows/nanoflows would be very welcome as well.
This will be one of the most user friendly change that Mendix will provide. Because while validating file uploader via Microflow, the confirmation pop-up comes before validation pop-up.