Media: Introduce image_editor_output_format filter

functions.php

<?php
  function my_theme_setup() {
    add_image_size( 'image-size-class', 1280, 720, true );
  }
  add_action( 'after_setup_theme', 'my_theme_setup' );

  function my_theme_wp_image_editor_output_format( $formats ) {
      $formats['image/jpg'] = 'image/webp';

      return $formats;
  }
  add_filter( 'image_editor_output_format', 'my_theme_wp_image_editor_output_format' );