🐛(back) validate document content in serializer

We recently extract images url in the content. For this, we assume that
the document content is always in base64. We enforce this assumption by
checking if it's a valide base64 in the serializer.

src/backend/core/tests/documents/test_api_documents_update.py

@@ -328,3 +328,22 @@ def test_api_documents_update_administrator_or_owner_of_another(via, mock_user_t
     other_document.refresh_from_db()
     other_document_values = serializers.DocumentSerializer(instance=other_document).data
     assert other_document_values == old_document_values
+
+
+def test_api_documents_update_invalid_content():
+    """
+    Updating a document with a non base64 encoded content should raise a validation error.
+    """
+    user = factories.UserFactory(with_owned_document=True)
+    client = APIClient()
+    client.force_login(user)
+
+    document = factories.DocumentFactory(users=[[user, "owner"]])
+
+    response = client.put(
+        f"/api/v1.0/documents/{document.id!s}/",
+        {"content": "invalid content"},
+        format="json",
+    )
+    assert response.status_code == 400
+    assert response.json() == {"content": ["Invalid base64 content."]}