🏷️(backend) add content-type to uploaded files

All the uploaded files had the content-type set
to `application/octet-stream`. It create issues
when the file is downloaded from the frontend
because the browser doesn't know how to handle
the file.
We now determine the content-type of the file
and set it to the file object.

src/backend/core/api/viewsets.py

@@ -605,7 +605,10 @@ class DocumentViewSet(
         key = f"{document.key_base}/{ATTACHMENTS_FOLDER:s}/{file_id!s}.{extension:s}"
 
         # Prepare metadata for storage
-        extra_args = {"Metadata": {"owner": str(request.user.id)}}
+        extra_args = {
+            "Metadata": {"owner": str(request.user.id)},
+            "ContentType": serializer.validated_data["content_type"],
+        }
         if serializer.validated_data["is_unsafe"]:
             extra_args["Metadata"]["is_unsafe"] = "true"