Merge pull request #9466 from galaxyproject/topic/options_filter_attribute_str

fix options_filter_attribute: needs to be str

lib/galaxy/tools/parameters/basic.py

@@ -1913,7 +1913,7 @@ class DataToolParameter(BaseDataToolParameter):
             ref = getattr(ref, attribute)
         if call_attribute:
             ref = ref()
-        return ref
+        return str(ref)
 
     def to_dict(self, trans, other_values={}):
         # create dictionary and fill default parameters