The default file uploader won’t help you there. Because there are no events and hence it would be difficult to refresh the context object.
I am not clear about your use case, but may be you can use just one button and then separate the logic in that microflow by using HasContents in a split.
You can use the FileDropper widget.
Hope this helps.
I assume you are using the file manager widget?
Since there is no on click/change event on this widget, you can upload the file in a pop-up page.
When the user then uploads the file and clicks save you can make the save button call a microflow which changes te hasContent boolean to true.
Hope this helps!