brought back terms testing as with_ is not only way to call and we cannot guarantee terms is a list otherwise.

lib/ansible/plugins/lookup/file.py

@@ -27,6 +27,9 @@ class LookupModule(LookupBase):
 
     def run(self, terms, variables=None, **kwargs):
 
+        if not isinstance(terms, list):
+            terms = [ terms ]
+
         ret = []
         for term in terms:
             basedir_path  = self._loader.path_dwim(term)