Use .file? instead of .exists? for file existence

Author: cacheflow

lib/fcm.rb

@@ -303,7 +303,7 @@ def credentials_error_msg(param)
   end
 
   def filename_or_io_like?(path)
-    (path.is_a?(String) || path.respond_to?(:open)) && File.exists?(path)
+    (path.is_a?(String) || path.respond_to?(:open)) && File.file?(path)
   end
 
   def json_key