Fix logging of file upload questions

src/pretix/control/views/orders.py

@@ -7,6 +7,7 @@ import pytz
 import vat_moss.id
 from django.conf import settings
 from django.contrib import messages
+from django.core.files import File
 from django.core.urlresolvers import reverse
 from django.db.models import Count
 from django.http import FileResponse, Http404, HttpResponseNotAllowed
@@ -698,7 +699,12 @@ class OrderModifyInformation(OrderQuestionsViewMixin, OrderView):
         self.invoice_form.save()
         self.order.log_action('pretix.event.order.modified', {
             'invoice_data': self.invoice_form.cleaned_data,
-            'data': [f.cleaned_data for f in self.forms]
+            'data': [{
+                k: (f.cleaned_data.get(k).name
+                    if isinstance(f.cleaned_data.get(k), File)
+                    else f.cleaned_data.get(k))
+                for k in f.changed_data
+            } for f in self.forms]
         }, user=request.user)
         if self.invoice_form.has_changed():
             success_message = ('The invoice address has been updated. If you want to generate a new invoice, '