Error parsing base64 strings in webhook

[tontan2545]

Hi, I've recently deployed a model in cog and I found quite an interesting issue.
If I'm using PUT /api/predictions/{prediction_id} the base64 result looks just fine, here's the example result.

However if I tried to do predictions in webhook manner, the base64 result came with as a JSON string content as file name but no base64 data.

I'm using Next.js API route for this webhook receiver