Retrieve MIME type from Base64 encoded String

In general, a base 64-encoded string could contain absolutely any data, so there is no way to know its file type.

To determine if it is an instance of a JPEG image, you'd need to base64-decode it, and then do something like checking its magic number, which is useful in telling you what the file isn't. You'd still need to do more work to determine if it is a valid JPEG image.

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAAMSURBVBhXY/j//z8ABf4C/qc1gYQAAAAASUVORK5CYII=

Is a sample image. Just split it with the first slash and get array index 1. Supposing the image is coming from a trusted client.