file_manager: use get_list WebRequest method

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

moonraker/components/file_manager/file_manager.py

@@ -619,11 +619,7 @@ class FileManager:
                     f"Cannot create archive at '{dest_path}'.  Parent "
                     "directory does not exist."
                 )
-            items: Union[str, List[str]] = web_request.get("items")
-            if isinstance(items, str):
-                items = [
-                    item.strip() for item in items.split(",") if item.strip()
-                ]
+            items = web_request.get_list("items")
             if not items:
                 raise self.server.error(
                     "At least one file or directory must be specified"