更新：
尝试从 Neckbeard 推荐的命令：
ffmpeg -analyzeduration 10M -probesize 10M -i Webpfile.webp -c:v libx264 output.mp4
这是输出（广义的文件路径）：
User-macbookpro:~ User$ ffmpeg -analyzeduration 10M -probesize 10M -i Webpfile.webp -c:v libx264 output.mp4
ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers
built with Apple LLVM version 9.0.0 (clang-900.0.38)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.4 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopenh264 --enable-libopus --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-opencl --enable-videotoolbox --disable-lzma --enable-libopenjpeg --disable-decoder=jpeg2000 --extra-cflags=-I/usr/local/Cellar/openjpeg/2.3.0/include/openjpeg-2.3 --enable-nonfree
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 …