check for destination and not source file extension when marking folder as existing

libconv.py

@@ -180,7 +180,7 @@ def prepare_destination(path: str) -> Tuple[str,bool]:
     new_path = DST_FOLDER + sub_path
     mkdir(new_path)
 
-    existing: bool = len(os.listdir(new_path)) != 0
+    existing: bool = len(get_files_with_ext(new_path, DST_FILE_EXT)) != 0
 
     for img in images:
         img_name = img.replace(path, "")