diff --git a/php/monoformat.php b/php/monoformat.php
index 777ed64..68f9edf 100644
--- a/php/monoformat.php
+++ b/php/monoformat.php
@@ -195,6 +195,8 @@ $section->elements = $elements;
 $file = new monoformat\File();
 array_push($file->sections, $section);
 
+Header("Content-Type: application/octet-stream");
+
 print($file->serialize());
 } // global