Order API: Add magic value to keep current file

doc/api/resources/orders.rst

@@ -1629,7 +1629,8 @@ Manipulating individual positions
    * ``answers``: If specified, you will need to provide **all** answers for this order position.
      Validation is handled the same way as when creating orders through the API. You are therefore
      expected to provide ``question``, ``answer``, and possibly ``options``. ``question_identifier``
-     and ``option_identifiers`` will be ignored.
+     and ``option_identifiers`` will be ignored. As a special case, you can submit the magic value
+     ``"file:keep"`` as the answer to a file question to keep the current value without re-uploading it.
 
    **Example request**:
 

src/pretix/api/serializers/order.py

@@ -168,6 +168,8 @@ class AnswerSerializer(I18nAwareModelSerializer):
         return q
 
     def _handle_file_upload(self, data):
+        if data['answer'] == 'file:keep':
+            return data
         try:
             ao = self.context["request"].user or self.context["request"].auth
             cf = CachedFile.objects.get(
@@ -439,6 +441,8 @@ class OrderPositionSerializer(I18nAwareModelSerializer):
                     if isinstance(answ_data['answer'], File):
                         a.file.save(answ_data['answer'].name, answ_data['answer'], save=False)
                         a.answer = 'file://' + a.file.name
+                    elif a.answer.startswith('file://') and answ_data['answer'] == "file:keep":
+                        pass  # keep current file
                     else:
                         for attr, value in answ_data.items():
                             setattr(a, attr, value)

src/tests/api/test_orders.py

@@ -4951,6 +4951,7 @@ def test_position_update_question_handling(token_client, organizer, event, order
     )
     assert r.status_code == 201
     file_id_png = r.data['id']
+
     payload = {
         'answers': [
             {
@@ -4971,3 +4972,24 @@ def test_position_update_question_handling(token_client, organizer, event, order
         answ = op.answers.get()
     assert answ.file
     assert answ.answer.startswith("file://")
+
+    payload = {
+        'answers': [
+            {
+                "question": question.id,
+                "answer": "file:keep"
+            }
+        ]
+    }
+    question.type = Question.TYPE_FILE
+    question.save()
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/'.format(
+            organizer.slug, event.slug, op.pk
+        ), format='json', data=payload
+    )
+    assert resp.status_code == 200
+    with scopes_disabled():
+        answ = op.answers.get()
+    assert answ.file
+    assert answ.answer.startswith("file://")