limit WPJM Applications allowed file types (untested)

gistfile1.txt

function restrict_wpjm_applications_mime_types( $allowed_mime_types, $field) {
    if ( $field['label'] == 'My Field' ) {
        unset( $allowed_mime_types['png'] );
        $allowed_mime_types['json'] = 'application/json';
    }

    return $allowed_mime_types;
}
add_filter( 'job_manager_mime_types', 'restrict_wpjm_applications_mime_types' );