Source code for https://victorfont.com/customize-formidable-forms-email-mime-type/

add_filter_wp_mail_content_type.php

<?php

add_filter( 'wp_mail_content_type', 'set_content_type' );
function set_content_type( $content_type ) {
  return 'text/x-gm-impdata';
}

adjust_my_email_header.php

<?php

add_filter('frm_pre_create_entry', 'adjust_my_email_header');
function adjust_my_email_header($values){
    if ( $values['form_key'] == 'contact2' ) {
        add_filter('frm_email_header', 'change_email_content_type');
    }
    return $values;
}

function change_email_content_type($header) {
    /* remove the Content Type element of the array */
    array_pop($header);
    /* needed to preserve compatibility */
    $charset      = get_option('blog_charset');
    $content_type = 'Content-Type: text/x-gm-impdata; charset="' . esc_attr( $charset ) . '"';
    $header[] = $content_type;
    return $header;
}

change_formidable_email_content_type.php

<?php

add_filter('frm_email_header', 'change_formidable_email_content_type');
function change_formidable_email_content_type($header) {
    /* remove the Content Type element of the array (the last element) */
    array_pop($header);
    /* needed to preserve compatibility */
    $charset = get_option('blog_charset');
    $content_type = 'Content-Type: text/x-gm-impdata; charset="' . esc_attr( $charset ) . '"';
    $header[] = $content_type;
    return $header;
}