How are the files being uploaded? Are you using the lightning:fileUpload standard component (https://developer.salesforce.com/docs/component-library/bundle/lightning:fileUpload/documentation)? If you're instantiating this component by code, you can pass in an onuploadfinished argument (a function). In my experience, this function executes when the modal is closed (that is, the confirmation modal resulting from the file upload). I don't think that specific behavior is documented by Salesforce anywhere, but I've found that to be the case.

Something like:

$A.createComponent(
   'lightning:fileUpload',
   {
      "label": "Test file upload",
      "onuploadfinished": function(){
         // *** Run logic for modal closed event here *** 
      }
   },
   function(newComponent, status){
      // Callback from component creation
   }
);

How are the files being uploaded? Are you using the lightning:fileUpload standard component (https://developer.salesforce.com/docs/component-library/bundle/lightning:fileUpload/documentation)? If you're instantiating this component by code, you can pass in an onuploadfinished argument (a function). In my experience, this function executes when the modal is closed (that is, the confirmation modal resulting from the file upload). According to Salesforce's documentation, this function runs "when the files have finished uploading", but I've found that it runs when the modal is closed (at least currently).

Something like:

$A.createComponent(
   'lightning:fileUpload',
   {
      "label": "Test file upload",
      "onuploadfinished": function(){
         // *** Run logic for modal closed event here *** 
      }
   },
   function(newComponent, status){
      // Callback from component creation
   }
);