（1）+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data;
通过传入的image的data数据解析出是什么格式的图片。

+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
    if (!data) {
        return SDImageFormatUndefined;
    }
    
    // File signatures table: http://www.garykessler.net/library/file_sigs.html
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return SDImageFormatJPEG;
        case 0x89:
            return SDImageFormatPNG;
        case 0x47:
            return SDImageFormatGIF;
        case 0x49:
        case 0x4D:
            return SDImageFormatTIFF;
        case 0x52: {
            if (data.length >= 12) {
                //RIFF....WEBP
                NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
                if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                    return SDImageFormatWebP;
                }
            }
            break;
        }
        case 0x00: {
            if (data.length >= 12) {
                //....ftypheic ....ftypheix ....ftyphevc ....ftyphevx
                NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(4, 8)] encoding:NSASCIIStringEncoding];
                if ([testString isEqualToString:@"ftypheic"]
                    || [testString isEqualToString:@"ftypheix"]
                    || [testString isEqualToString:@"ftyphevc"]
                    || [testString isEqualToString:@"ftyphevx"]) {
                    return SDImageFormatHEIC;
                }
                //....ftypmif1 ....ftypmsf1
                if ([testString isEqualToString:@"ftypmif1"] || [testString isEqualToString:@"ftypmsf1"]) {
                    return SDImageFormatHEIF;
                }
            }
            break;
        }
    }
    return SDImageFormatUndefined;
}

• 文件头规范
  JPEG (jpg)，文件头：FFD8FFE1
  PNG (png)，文件头：89504E47
  GIF (gif)，文件头：47494638
  TIFF tif;tiff 0x49492A00
  TIFF tif;tiff 0x4D4D002A
  WebP : 524946462A73010057454250

首先拿出data的第一个字节的东西，然后去判断是什么类型。这还有个链接，http://www.garykessler.net/library/file_sigs.html关于文件前缀类型的

下面俩方法就是枚举和字符串互相转换的

• (nonnull CFStringRef)sd_UTTypeFromImageFormat:(SDImageFormat)format;

• (SDImageFormat)sd_imageFormatFromUTType:(nonnull CFStringRef)uttype;