🔧(backend) settings CONVERSION_UPLOAD_ENABLED to control docspec usage

We want to control the conversion of document at upload time. We want to
disable this feature using a settings. The new settings
CONVERSION_UPLOAD_ENABLED should be used to enable or not the conversion
at upload feature. If disabled and a file is uploaded, the reponse will
return a 400

src/backend/core/tests/external_api/test_external_api_documents.py

@@ -261,7 +261,7 @@ def test_external_api_documents_create_subdocument_reader_not_allowed(
 
 @patch("core.services.converter_services.Converter.convert")
 def test_external_api_documents_create_with_markdown_file_success(
-    mock_convert, user_token, resource_server_backend, user_specific_sub
+    mock_convert, user_token, resource_server_backend, user_specific_sub, settings
 ):
     """
     Users with an access token should be able to create documents through the resource
@@ -272,6 +272,8 @@ def test_external_api_documents_create_with_markdown_file_success(
     client = APIClient()
     client.credentials(HTTP_AUTHORIZATION=f"Bearer {user_token}")
 
+    settings.CONVERSION_UPLOAD_ENABLED = True
+
     # Mock the conversion
     converted_yjs = "base64encodedyjscontent"
     mock_convert.return_value = converted_yjs