[experiments][admin] Fixed editing

beat/web/experiments/admin.py

@@ -112,7 +112,10 @@ class ExperimentModelForm(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
+
 
 
 class BlockInline(admin.TabularInline):