avthumb参数formats格式及解释
https://developer.qiniu.com/dora/api/audio-and-video-transcoding-avthumb
参数 format：目标音频（比如 mp3 m4a 等）或视频（比如 avi 、flv 、mp4等）格式。
该参数支持具体格式如下表：
| formats | description |
|---|---|
| 3g2 | 3GP2 (3GPP2 file format) |
| 3gp | 3GP (3GPP file format) |
| a64 | a64 video for Commodore 64 |
| ac3 | raw AC-3 |
| adts | ADTS AAC (Advanced Audio Coding) |
| adx | CRI ADX |
| aiff | Audio IFF |
| alaw | PCM A-law |
| amr | 3GPP AMR |
| asf | ASF (Advanced / Active Streaming Format) |
| asf_stream | ASF (Advanced / Active Streaming Format) |
| ass | SSA (SubStation Alpha) subtitle |
| ast | AST (Audio Stream) |
| au | Sun AU |
| avi | AVI (Audio Video Interleaved) |
| avm2 | SWF (ShockWave Flash) (AVM2) |
| bit | G.729 BIT file format |
| caf | Apple CAF (Core Audio Format) |
| cavsvideo | raw Chinese AVS (Audio Video Standard) video |
| crc | CRC testing |
| data | raw data |
| daud | D-Cinema audio |
| dirac | raw Dirac |
| dnxhd | raw DNxHD (SMPTE VC-3) |
| dts | raw DTS |
| dv | DV (Digital Video) |
| dvd | MPEG-2 PS (DVD VOB) |
| eac3 | raw E-AC-3 |
| f32be | PCM 32-bit floating-point big-endian |
| f32le | PCM 32-bit floating-point little-endian |
| f4v | F4V Adobe Flash Video |
| f64be | PCM 64-bit floating-point big-endian |
| f64le | PCM 64-bit floating-point little-endian |
| ffm | FFM (FFserver live feed) |
| ffmetadata | FFmpeg metadata in text |
| filmstrip | Adobe Filmstrip |
| flac | raw FLAC |
| flv | FLV (Flash Video) |
| framecrc | framecrc testing |
| framemd5 | Per-frame MD5 testing |
| g722 | raw G.722 |
| g723_1 | raw G.723.1 |
| gif | GIF Animation |
| gxf | GXF (General eXchange Format) |
| h261 | raw H.261 |
| h263 | raw H.263 |
| h264 | raw H.264 video |
| hls | Apple HTTP Live Streaming |
| ico | Microsoft Windows ICO |
| ilbc | iLBC storage |
| image2 | image2 sequence |
| image2pipe | piped image2 sequence |
| ipod | iPod H.264 MP4 (MPEG-4 Part 14) |
| ircam | Berkeley/IRCAM/CARL Sound Format |
| ismv | ISMV/ISMA (Smooth Streaming) |
| ivf | On2 IVF |
| jacosub | JACOsub subtitle format |
| latm | LOAS/LATM |
| m4v | raw MPEG-4 video |
| matroska | Matroska |
| md5 | MD5 testing |
| microdvd | MicroDVD subtitle format |
| mjpeg | raw MJPEG video |
| mkvtimestamp_v2 | extract pts as timecode v2 format, as defined by mkvtoolnix |
| mlp | raw MLP |
| mmf | Yamaha SMAF |
| mov | QuickTime / MOV |
| mp2 | MP2 (MPEG audio layer 2) |
| mp3 | MP3 (MPEG audio layer 3) |
| m4a | M4A (MPEG-4 audio ) |
| mp4 | MP4 (MPEG-4 Part 14) |
| mpeg | MPEG-1 Systems / MPEG program stream |
| mpeg1video | raw MPEG-1 video |
| mpeg2video | raw MPEG-2 video |
| mpegts | MPEG-TS (MPEG-2 Transport Stream) |
| mpjpeg | MIME multipart JPEG |
| mulaw | PCM mu-law |
| mxf | MXF (Material eXchange Format) |
| mxf_d10 | MXF (Material eXchange Format) D-10 Mapping |
| nut | NUT |
| ogg | Ogg |
| oma | Sony OpenMG audio |
| psp | PSP MP4 (MPEG-4 Part 14) |
| rawvideo | raw video |
| rm | RealMedia |
| roq | raw id RoQ |
| rso | Lego Mindstorms RSO |
| rtp | RTP output |
| rtsp | RTSP output |
| s16be | PCM signed 16-bit big-endian |
| s16le | PCM signed 16-bit little-endian |
| s24be | PCM signed 24-bit big-endian |
| s24le | PCM signed 24-bit little-endian |
| s32be | tPCM signed 32-bit big-endian |
| s32le | PCM signed 32-bit little-endian |
| s8 | PCM signed 8-bit |
| sap | SAP output |
| segment | segment |
| smjpeg | Loki SDL MJPEG |
| smoothstreaming | Smooth Streaming Muxer |
| sox | SoX native |
| spdif | IEC 61937 (used on S/PDIF - IEC958) |
| srt | SubRip subtitle |
| stream_segment | streaming segment muxer |
| ssegment | streaming segment muxer |
| svcd | MPEG-2 PS (SVCD) |
| swf | SWF (ShockWave Flash) |
| tee | Multiple muxer tee |
| truehd | raw TrueHD |
| u16be | PCM unsigned 16-bit big-endian |
| u16le | PCM unsigned 16-bit little-endian |
| u24be | PCM unsigned 24-bit big-endian |
| u24le | PCM unsigned 24-bit little-endian |
| u32be | PCM unsigned 32-bit big-endian |
| u32le | PCM unsigned 32-bit little-endian |
| u8 | PCM unsigned 8-bit |
| vc1 | raw VC-1 video |
| vc1test | VC-1 test bitstream |
| vcd | MPEG-1 Systems / MPEG program stream (VCD) |
| vob | MPEG-2 PS (VOB) |
| voc | Creative Voice |
| w64 | Sony Wave64 |
| wav | WAV / WAVE (Waveform Audio) |
| webm | Matroska / WebM |
| webvtt | WebVTT subtitle |
| wtv | Windows Television (WTV) |
| wv | raw WavPack |
| yuv4mpegpipe | YUV4MPEG pipe |
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