diff --git a/openhtf/output/servers/station_server.py b/openhtf/output/servers/station_server.py
index 690edd523..634beb3af 100644
--- a/openhtf/output/servers/station_server.py
+++ b/openhtf/output/servers/station_server.py
@@ -360,7 +360,8 @@ def get(self, test_uid, phase_descriptor_id, attachment_name):
       self.set_status(404)
       return
 
-    self.set_header('Content-Type', attachment.mimetype)
+    if attachment.mimetype:
+      self.set_header('Content-Type', attachment.mimetype)
     self.write(attachment.data)