storage-contact-form-7-submission-preview.php

<?php

add_filter( 'cf7_entry_rows', function( $rows, $entry ) {

	$attachments = get_post_meta( $entry->ID, 'mail_attachments', true );

	if ( empty( $attachments ) ) {
		return $rows;
	}

	$uploads_dir = wp_upload_dir();
	$storage_url = sprintf( '%s/cf7-storage', $uploads_dir['baseurl'] );
	$attachment_list = array();

	foreach ( $attachments as $path ) {

		$extension = pathinfo( $path, PATHINFO_EXTENSION );
		$link = sprintf( '%s/%d-%s.%s', $storage_url, $entry->ID, md5( $path ), $extension );

		// Render as a preview
		if ( in_array( $extension, array( 'jpeg', 'jpg', 'png', 'gif' ) ) ) {
			$preview = sprintf(
				'<img src="%s" alt="" width="300" />',
				$link
			);
		} else {
			$preview = esc_html( basename( $path ) );
		}

		$attachment_list[] = sprintf(
			'<li><a href="%s" target="_blank">%s</a></li>',
			esc_url( $link ),
			$preview
		);

	}

	$rows['attachments']['value'] = sprintf(
		'<ul>%s</ul>',
		implode( '', $attachment_list )
	);

	return $rows;

}, 10, 2 );