Fix sprite upload on empty expression

- When no sprite was defined before, it falsely tried to derive the filename from the existing filename when "allow multiple" was not enabled. It now correctly just utilizes the expression name, not relying on filename anymore.

public/scripts/extensions/expressions/index.js

@@ -1810,7 +1810,7 @@ async function onClickExpressionUpload(event) {
                 }
             }
         } else {
-            spriteName = withoutExtension(clickedFileName);
+            spriteName = withoutExtension(expression);
         }
 
         if (!spriteName) {