Same thing happened in November last year.
Here’s the error response in JSON:

{“data”:{“error”:{“code”:1003,“message”:“File type invalid”,“type”:“ImgurException”,“exception”:[]},“request”:"/3/image",“method”:“POST”},“success”:false,“status”:400}

People tend to try it multiple times if it’s failed to upload an image.
The requests are rising and it’s counting on our quotas.
So please fix it ASAP.

Thanks!