diff --git a/beat/web/dataformats/admin.py b/beat/web/dataformats/admin.py
index 0a4487983d2f449d80fb90f2a6b4dd4f45aa6f74..9fbf1d4c10127b348b49fa03ca6ea871220706a6 100644
--- a/beat/web/dataformats/admin.py
+++ b/beat/web/dataformats/admin.py
@@ -123,7 +123,10 @@ class DataFormatModelForm(forms.ModelForm):
 
         if 'declaration_file' in self.data and \
                 isinstance(self.data['declaration_file'], six.string_types):
-            self.data['declaration_file'] = ContentFile(self.data['declaration_file'], name='unsaved')
+            mutable_data = self.data.copy()
+            mutable_data['declaration_file'] = ContentFile(self.data['declaration_file'], name='unsaved')
+            self.data = mutable_data
+
 
 
 #----------------------------------------------------------