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

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         body:
-          - README.md
+          - file://README.md
           - Test of action completed successfully!
     steps:
       - name: Checkout

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
 ```

main.rb

@@ -14,7 +14,13 @@ to = ENV['INPUT_TO']
 from = ENV['INPUT_FROM']
 
 # Body
-body = File.exist?(body) ? File.read(body) : body
+prefix = 'file://'
+body = if body.start_with?(prefix)
+         path = body.delete_prefix(prefix)
+         File.read(path)
+       else
+         body
+       end
 
 # Message
 message = <<~END_OF_MESSAGE