See: https://calderaforms.com/doc/caldera_forms_mailer/

custom-caldera-html-email.php

<?php
/**
* Use a custom HTML file for your Caldera Forms email.
*/
add_filter( 'caldera_forms_mailer', function ( $mail, $data, $form ) {
	//MAKE SURE TO CHANGE FORM ID
	if ( 'CF1243..' == $form[ 'ID' ] ) {
		ob_start();
		//MAKE SURE TO CHANGE THIS TO YOUR FILE PATH
		 //BTW - You have all submission data in $data if you want to use in your html file
		 include '/path_to_file.html';
		$html = ob_get_clean();
		//Optional - Substitute Caldera Forms magic tags.
		$html                = Caldera_Forms::do_magic_tags( $html );
		$mail[ 'message' ] = $html;
	}

	return $mail;
}, 10, 3 );