convert_activestorage_to_mp3.rb

thing.audio.open do |file|
  input = file.path
  output = "#{file.path}.mp3"
  filename = "#{thing.audio.filename}.mp3"

  system("ffmpeg", "-i", input, output, exception: true)
  thing.audio_mp3.attach(io: File.open(output), filename:, content_type: "audio/mpeg")
ensure
  File.delete(output) if File.exist?(output)
end

transcode_audio_job.rb

class TranscodeAudioJob < ApplicationJob
  def perform(input_attachment, target, output_attribute, extension, content_type = Mime::Type.lookup_by_extension(extension).to_s)
    output_attachment = target.send(output_attribute)

    input_attachment.open do |file|
      input = file.path
      output = "#{file.path}.#{extension}"
      filename = "#{input_attachment.filename}.#{extension}"

      system("ffmpeg", "-i", input, output, exception: true)
      output_attachment.attach(io: File.open(output), filename:, content_type:)
    ensure
      File.delete(output) if File.exist?(output)
    end
  end
end