diff --git a/openmc/source.py b/openmc/source.py
index fdb2d5bc25..48e14f6f71 100644
--- a/openmc/source.py
+++ b/openmc/source.py
@@ -223,10 +223,7 @@ class Source:
         if self.particle != 'neutron':
             element.set("particle", self.particle)
         if self.file is not None:
-            if (self.file.endswith('.mcpl')):
-              element.set("mcpl", self.file)
-            else:
-              element.set("file", self.file)
+            element.set("file", self.file)
         if self.library is not None:
             element.set("library", self.library)
         if self.parameters is not None: