There are two ways to accomplish this:
The easy way is if your drop down contains an enumeration, then you can make the file upload widget conditionally visible based on the value of the enumeration.
If you have a reference to an object, you can use an on change microflow on the reference selector. First, add a boolean to the entity to enable you to make the file upload widget conditionally visible. Then, in the on change microflow, if the value of the drop down has been changed, you can set a boolean to true, to show the file upload widget.