判断图片格式

+(NSString *)typeForImageData:(NSData *)data
{
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return @"jpeg";
            
        case 0x89:
            return @"png";
            
        case 0x47:
            return @"gif";
            
        case 0x49:
            
        case 0x4D:
            return @"tiff";
    }
    return nil;
}