Changing a file's extension

My goal is simple:
If there's a TXT file in the watched folder
Then Change the extension to MD
That's all I want.
I have a feeling that this should be rather obvious, since it's nothing but renaming. Still, my rule didn't work:
If ALL conditions are met for THE FILE OR FOLDER BEING MATCHED:
NAME matches (•Name).txt
Do
RENAME with (•Name).md
(with •Name as an Anything element.)
When I click the "Preview" button (bottom of the folder list, the txt file wasn't matched to that rule at all.
Thanks for your help.
If there's a TXT file in the watched folder
Then Change the extension to MD
That's all I want.
I have a feeling that this should be rather obvious, since it's nothing but renaming. Still, my rule didn't work:
If ALL conditions are met for THE FILE OR FOLDER BEING MATCHED:
NAME matches (•Name).txt
Do
RENAME with (•Name).md
(with •Name as an Anything element.)
When I click the "Preview" button (bottom of the folder list, the txt file wasn't matched to that rule at all.
Thanks for your help.