You can use magic numbers to detect a MIME type (check here the list of file signatures). However, file signatures are not 100% reliable and you can easily encounter false positives. Of course, there are tasks when a such solution is more than enough.
So if you have a Base64 string and want to identify its MIME type using file signatures you don’t need to decode the Base64. A much faster way is to store the file signatures as Base64 and just check if input starts with one of them. A simple example:
var signatures = {
JVBERi0: "application/pdf",
R0lGODdh: "image/gif",
R0lGODlh: "image/gif",
iVBORw0KGgo: "image/png",
"/9j/": "image/jpg"
};
function detectMimeType(b64) {
for (var s in signatures) {
if (b64.indexOf(s) === 0) {
return signatures[s];
}
}
}
// Some tests
console.log(detectMimeType('R0lGODdhAQABAPAAAP8AAAAAACwAAAAAAQABAAACAkQBADs="));
console.log(detectMimeType("iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAADElEQVR42mP4z8AAAAMBAQD3A0FDAAAAAElFTkSuQmCC'));
console.log(detectMimeType('JVBERi0xLjUKJYCBgoMKMSAwIG9iago8PC9GaWx0ZXIvRmxhdGVEZWNvZGUvRmlyc3QgMTQxL04gMjAvTGVuZ3'));
console.log(detectMimeType('/9j/4AAQSkZJRgABAQAAZABkAAD/2wCEABQQEBkSGScXFycyJh8mMi4mJiYmLj41NTU1NT5EQUFBQUFBRERERERERERE'));