How to make sure the uploaded file is really an image?

Uploadcare checks file content to approve that the uploaded file is an image. If you see is_image flag set to true, you can be sure it is an image that Uploadcare CDN can work with.
See how it works here.

Note that we don’t provide on-the-fly image transformations for images greater than 75 Mpx in resolution: those can only be delivered via CDN as-is. See this article to learn more about the limits of our CDN

Also, you have access to our REST API and you can check file properties (without the need to get actual file). Also, you can validate files before uploading to make sure that they fit into the limit.