no that doesn’t exists. What you can do is validate the property $YourFileDocumentObject/HasContents in a microflow. This is a Boolean, if it returns true a file is uploaded.

//Edit//

The microflow triggered by the save button would look like this.

As per your description I assume you need to give some validation message to user when he/she presses the upload document button but no document is selected.

You can use HasContents attribute of the FileDocument entity. If it is false then you know that there is no document present and hence show necessary message to the user.

Unfortunately not out of the box… Something so basic as this.

Tom Worthington has written a nice tutorial to get it set up properly:

Mendix UI hack: fix file upload validation in five easy steps! | AuraQ