[plotters][admin] Fix immutable query object modification error

Work now on copy of QueryObject as recommended in the Django
documentation and replace the original with the copy

beat/web/plotters/admin.py

@@ -27,11 +27,10 @@
 
 '''Administrative add-on for the extended Django User model'''
 
-import six
-
 from django.contrib import admin
 from django import forms
 from django.core.files.base import ContentFile
+from django.utils import six
 
 from ..common.texts import Messages
 
@@ -134,7 +133,9 @@ class PlotterModelForm(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
 
 
 #----------------------------------------------------------