Unable to find a suitable output format for 'ffmpeg' :Invalid argument

I’m trying to mix two audio files using flutter_ffmpeg: 0.2.10 but I get

[NULL @ 0xd77d4400] Unable to find a suitable output format for 'ffmpeg'
I/flutter (29205): ffmpeg: Invalid argument

code : 
String file1= _localPath+"/file1.m4a";
String file2= _localPath+"/file2.mp3";
String output= _localPath+"/abhijit.wav";

String finalPath= "ffmpeg -i "+ file1 +" -i "+file2+" -filter_complex amix=inputs=2:duration=first:dropout_transition=0 "+output;

console ffmpeg's message...
I/flutter (29205):   libavutil      56. 38.100 / 56. 38.100
I/flutter (29205):   libavcodec     58. 65.102 / 58. 65.102
I/flutter (29205):   libavformat    58. 35.101 / 58. 35.101
I/flutter (29205):   libavdevice    58.  9.103 / 58.  9.103
I/flutter (29205):   libavfilter     7. 70.101 /  7. 70.101
I/flutter (29205):   libswscale      5.  6.100 /  5.  6.100
I/flutter (29205):   libswresample   3.  6.100 /  3.  6.100
I/flutter (29205): Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/storage/emulated/0/Android/data/com.arthenica.flutter.ffmpeg.FlutterFFmpegExample/files/Download/file1.m4a':
I/flutter (29205):   Metadata:
I/flutter (29205):     major_brand     : mp42
I/flutter (29205):     minor_version   : 1
I/flutter (29205):     compatible_brands: isommp41mp42
I/flutter (29205):     creation_time   : 2020-07-09T15:42:47.000000Z
I/flutter (29205):   Duration: 00:12:12.26, start: 0.000000, bitrate: 61 kb/s
I/flutter (29205):     Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 60 kb/s (default)
I/flutter (29205):     Metadata:
I/flutter (29205):       creation_time   : 2020-07-09T15:42:47.000000Z
I/flutter (29205):       handler_name    : Core Media Audio
I/flutter (29205): Input #1, mp3, from '/storage/emulated/0/Android/data/com.arthenica.flutter.ffmpeg.FlutterFFmpegExample/files/Download/file2.mp3':
I/flutter (29205):   Metadata:
I/flutter (29205):     artist          : SoundHelix
I/flutter (29205):     genre           : Techno
I/flutter (29205):     encoder         : Lavf58.20.100
I/flutter (29205):   Duration: 00:00:23.12, start: 0.011995, bitrate: 192 kb/s
I/flutter (29205):     Stream #1:0: Audio: mp3, 44100 Hz, stereo, fltp, 192 kb/s
I/flutter (29205):     Metadata:
I/flutter (29205):       encoder         : Lavf
I/flutter (29205): [NULL @ 0xd77d4400] Unable to find a suitable output format for 'ffmpeg'
I/flutter (29205): ffmpeg: Invalid argument
I/flutter (29205): "
I/flutter (29205): New FFmpeg pipe: /data/user/0/com.arthenica.flutter.ffmpeg.FlutterFFmpegExample/cache/mf_pipe_7
I/flutter (29205): FFmpeg process exited with rc 1

Please check command of FFmpeg and suggest me if any correction occur

String finalPath= "-i "+ file1 +" -i "+file2+" -filter_complex amix=inputs=2:duration=first:dropout_transition=0 "+output;