🐛(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/api/serializers.py

@@ -1,6 +1,8 @@
 """Client serializers for the impress core app."""
 
+import binascii
 import mimetypes
+from base64 import b64decode
 
 from django.conf import settings
 from django.db.models import Q
@@ -299,6 +301,18 @@ class DocumentSerializer(ListDocumentSerializer):
 
         return value
 
+    def validate_content(self, value):
+        """Validate the content field."""
+        if not value:
+            return None
+
+        try:
+            b64decode(value, validate=True)
+        except binascii.Error as err:
+            raise serializers.ValidationError("Invalid base64 content.") from err
+
+        return value
+
     def save(self, **kwargs):
         """
         Process the content field to extract attachment keys and update the document's