The upload button only works if the Mendix object actually exists.
You could replace the standard 'New' button on the datagrid with a microflow button that creates a new employee and employee image object, and links them together without commit.
Then display your page and the upload should work.
You also need a custom save button because you need to commit the image as well as the employee object. Your save button microflow will get the employee as parameter, retrieve the employee image then commit both.