Command :

@Echo off
Echo "*****************START***************"
FOR %%G in (*.webm) DO (
  "ffmpeg\bin\ffmpeg" -i "%%G" -codec copy "%%~nG.mp4")
Echo "*****************END***************"
pause

Console output :

"*****************START***************"
ffmpeg version N-44818-g13f0cd6 Copyright (c) 2000-2012 the FFmpeg developers
  built on Sep 27 2012 19:30:20 with gcc 4.7.1 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-pthreads --enable-runt
ime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass -
-enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-l
ibfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenj
peg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheo
ra --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-li
bvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --ena
ble-zlib
  libavutil      51. 73.101 / 51. 73.101
  libavcodec     54. 59.100 / 54. 59.100
  libavformat    54. 29.104 / 54. 29.104
  libavdevice    54.  2.101 / 54.  2.101
  libavfilter     3. 17.100 /  3. 17.100
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, matroska,webm, from 'Bande annonce.webm':
  Duration: 00:03:07.40, start: 0.000000, bitrate: 3114 kb/s
    Stream #0:0: Video: vp8, yuv420p, 1280x720, SAR 1:1 DAR 16:9, 29.97 fps, 29.
97 tbr, 1k tbn, 1k tbc (default)
    Stream #0:1: Audio: vorbis, 44100 Hz, stereo, s16 (default)
[mp4 @ 040605e0] track 0: could not find tag, codec not currently supported in c
ontainer
Output #0, mp4, to 'Bande annonce.mp4':
  Metadata:
    encoder         : Lavf54.29.104
    Stream #0:0: Video: vp8, yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 29.97
 fps, 90k tbn, 1k tbc (default)
    Stream #0:1: Audio: vorbis ([221][0][0][0] / 0x00DD), 44100 Hz, stereo (defa
ult)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Could not write header for output file #0 (incorrect codec parameters ?): Operat
ion not permitted
"*****************END***************"
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