You need to add the code in your themes functions.php, may be at the bottom of that file.

WordPress doesn't support ai/eps extensions to be uploaded. So you need to make the support too. Below is the full code for extension support, both for plugin and WP:

[php]
function wpufe_custom_upload_extensions( $extensions ) {
$extensions[‘custom'] = array(
‘label' => __( ‘Custom Extensions', ‘wpuf' ),
‘ext' => ‘eps,ai',
);

return $extensions;
}

add_filter( ‘wpuf_allowed_extensions', ‘wpufe_custom_upload_extensions' );

function custom_upload_mimes ( $existing_mimes ) {
$existing_mimes[‘ai'] = ‘application/pdf';
$existing_mimes[‘eps'] = ‘application/octet-stream';

return $existing_mimes;
}

add_filter(‘upload_mimes', ‘custom_upload_mimes');
[/php]

There will be a new option (Custom Extensions) will be added in the form editors file upload field. You've to enable it too.