Dynamically add file attachments to Contact Form 7 emails from a custom field

contact-form-7-dynamic-attachment.php

<?php
/* Dynamically add file attachments to Contact Form 7 emails from a Wordpress custom field.
 * Custom field is 'case-pdf' in the example.
 */

add_action('wpcf7_before_send_mail', 'wpcf7_add_attachment');

function wpcf7_add_attachment($contact_form) {
  global $_POST;
  $submission = WPCF7_Submission::get_instance();

  if ( $submission && !empty($_POST['case-pdf']) ) {
    $mail = $contact_form->prop('mail');
    $mail['attachments'] .= $_POST['case-pdf'];
    $contact_form->set_properties(array('mail' => $mail));
  }
}

Oof, this was a tricky one. I expected that WPCF7 would allow dynamic fields as mail tags in the attachments field out-of-the-box, but I had to jump through some hoops to make it work. If anyone has any feedback on how to do this more efficiently, I'd gladly hear so. Hope this helps :-)

The form contains a hidden field called whitepaper, as defined in WPCF7.

[hidden whitepaper default:shortcode_attr]

This is then rendered to a shortcode to populate the field with the value, in your template

// components/some-form-block.php
<?php
echo do_shortcode("[contact-form-7 id=123 whitepaper=uploads/2018/09/some-file-name.pdf]")
?>

To make sure the field actually gets populated, you'll need a snippet in your functions.php as well

// functions.php
add_filter('shortcode_atts_wpcf7', 'custom_shortcode_atts_wpcf7_filter', 10, 3);
function custom_shortcode_atts_wpcf7_filter($out, $pairs, $atts)
{
    $shortcodes = ['another-hidden-field', 'whitepaper'];

    foreach ($shortcodes as $shortcode) {
        if (isset($atts[$shortcode])) {
            $out[$shortcode] = $atts[$shortcode];
        }
    }

    return $out;
}

Finally, to add the attachment to the mail, another piece of code is needed in functions.php

// functions.php

/**
 * Custom piece of code to add a 'whitepaper' shortcode value
 * into the attachments of WPCF7. Using a [whitepaper] tag in 
 * the WPCF7 mail field does not work as expected. Perhaps because
 * 'whitepaper' is a hidden field and needs to be provided through shortcodes, like:
 * [contact-form-7 id=123 whitepaper=uploads/2021/12/path-to-file.pdf]
 */
add_action('wpcf7_before_send_mail', 'wpcf7_add_attachment');
function wpcf7_add_attachment($contact_form)
{
    global $_POST;
    $submission = WPCF7_Submission::get_instance();

    if ($submission && !empty($_POST['whitepaper'])) {

        $mail = $contact_form->prop('mail');
        $mail['attachments'] .= "\n{$_POST['whitepaper']}"; // Note the newline character here
        $contact_form->set_properties(array('mail' => $mail));
    }
}

I've been ending on the exact same solution :-)
I should have found this post earlier !