[FFmpeg-user] How to convert m4a to mp3 with thumbnail included?

[Sendex Six]

I have an m4a audio, and I want to convert this into mp3 with a thumbnail
included, and I pipe the result to an s3 bucket.

I can able to do converting, but the thumbnail was always ignored; I think
it is because of the pipe:1 or -

Because when I do it in the terminal and directly output it in my local
disk, it works, but it will not work using a pipe:1 or -.

How to include a thumbnail in the audio and directly stream the result to
the s3 bucket?

Thank you very much.