require file:// prefix in body if loading a file

README.md

@@ -13,10 +13,10 @@ An action that simply sends a mail to multiple recipients.
     username: ${{secrets.MAIL_USERNAME}}
     password: ${{secrets.MAIL_PASSWORD}}
     subject: Github Actions job result
-    # Literal body
+    # Literal body:
     body: Build job of ${{github.repository}} completed successfully!
     # Read file contents as body:
-    body: README.md
+    body: file://README.md
     to: obiwan@tatooine.com,yoda@dagobah.com
     from: Luke Skywalker
 ```