FFmpeg on Android - Unable to find a suitable output format

FFmpeg on Android - Unable to find a suitable output format

I want to reduce the resolution of a video before edit it, so I´m using
ffmpeg on android. I´m using the executable/binary file of ffmpeg and I´m
calling my ffmpeg commands like that:
./ffmpeg -i /sdcard/dcim/video.mp4 -s 320x240 -r 10 -y /sdcard/output.mp4
But when I try this command I get this error:
Unable to find a suitable output format for /sdcard/output.mp4
This is the output when I use the adb shell:
The only action I have been able to do is to extract the audio of a video
and save it as a mp3 using this command:
./ffmpeg -i /sdcard/dcim/video.mp4 -y /sdcard/output.mp3
That works well but this is the only action I can do... Any idea of why I
can´t convert a video? (I can´t copy it too) Thanks!!