diff --git a/planemo/galaxy/test/actions.py b/planemo/galaxy/test/actions.py
index 1b9777e6d..856f3d29f 100644
--- a/planemo/galaxy/test/actions.py
+++ b/planemo/galaxy/test/actions.py
@@ -125,16 +125,19 @@ def run_in_config(ctx, config, run=run_galaxy_command, test_data_target_dir=None
     ec = test_results.exit_code
     structured_data = test_results.structured_data
     extra_files = []
-    # check if tools created extra files in the file_path
+    # check if tools created extra files in the file_path all files in the files path
+    # - end with '.dat' or
+    # - are contained in a directory `dataset_.*_files`
     for (path, dirs, files) in os.walk(config.env["GALAXY_CONFIG_OVERRIDE_FILE_PATH"]):
         for name in files:
-            if not (name.endswith(".dat") or re.match('dataset_.*_files', name)):
-                extra_files += os.path.join(path, name)
+            if not (name.endswith(".dat") or re.match('.*/dataset_.*_files$', path)):
+                extra_files.append(os.path.join(path, name))
     if len(extra_files) > 0:
+        msg = f"One of the tested tools wrote to Galaxy's files dir: {extra_files}"
         for i in range(len(structured_data["tests"])):
             structured_data["tests"][i]["data"]["status"] = "failure"
-            structured_data["tests"][i]["data"]["job"]["stderr"] += "One of the tested tools wrote to Galaxy's files dir"
-        error("One of the tested tools wrote to Galaxy's files dir")
+            structured_data["tests"][i]["data"]["job"]["stderr"] += msg
+        error(msg)
         ec = 1
     return handle_reports_and_summary(
         ctx,