A quick function for determining an image's file type by its first couple of bytes

UTTypeForImageData.m

@import MobileCoreServices;

static CFStringRef UTTypeForImageData(NSData *data) {
    const unsigned char * bytes = [data bytes];

    if (data.length >= 8) {
        if (bytes[0] == 0x89 && bytes[1] == 0x50 && bytes[2] == 0x4E && bytes[3] == 0x47 && bytes[4] == 0x0D && bytes[5] == 0x0A && bytes[6] == 0x1A && bytes[7] == 0x0A) {
            return kUTTypePNG;
        }
    }

    if (data.length >= 4) {
        if (bytes[0] == 0xFF && bytes[1] == 0xD8 && bytes[2] == 0xFF && bytes[3] == 0xE0) {
            return kUTTypeJPEG;
        }
    }

    if (data.length >= 3) {
        if (bytes[0] == 0x47 && bytes[1] == 0x49 && bytes[2] == 0x46) {
            return kUTTypeGIF;
        } else if (bytes[0] == 0x49 && bytes[1] == 0x49 && bytes[2] == 0x2A) {
            return kUTTypeBMP;
        }
    }

    if (data.length >= 2) {
        if (bytes[0] == 0x42 && bytes[1] == 0x4D) {
            return kUTTypeBMP;
        }
    }

    return nil;
}

static CFStringRef UTTypeForImageData(NSData *data) {
    CFStringRef type = NULL;
    CGImageSourceRef isrc = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    if (isrc) {
        type = CGImageSourceGetType(isrc);
        CFRelease(isrc);
    }
    return type;
}

@kongtomorrow Touché!

For those of you following along at home, be sure to add:

@import ImageIO;

this is handy in js too

See also here: https://github.com/sdsykes/fastimage/blob/master/lib/fastimage.rb#L351