PDB File Selector:OnChange:enable/disable Upload button when set/not set

On Change of PDB File Selector Control:
Enable "Upload ..." button when a file is selected
Disable "Upload ..." button when a file is not selected

proxl_web_app/front_end/src/js/page_js/data_pages/project_search_ids_driven_pages/structure_page/structure-viewer-pdb-upload.js

@@ -109,8 +109,19 @@ function attachPDBFileUploadHandlers() {
 
 	// enable upload button if they select a file
 	$fileField.change( function( eventObject ) {
-
-		$uploadButton.prop("disabled", false );
+
+		// get file to upload
+		var fileToUpload = document.getElementById("pdb-file-field").files[ 0 ];
+
+		if ( fileToUpload ) {
+
+			//  Have file to upload
+			$uploadButton.prop("disabled", false );
+		} else {
+			// No file to upload
+			$uploadButton.prop("disabled", true );
+		}
+
 	});
 
 	// upload button defaults to disabled