WP Custom Upload MIME types

functions.php

<?php

function my_custom_mime_types( $mimes ) {
  // New allowed mime types.
  $mimes['svg'] = 'image/svg+xml';
  $mimes['svgz'] = 'image/svg+xml';
  $mimes['doc'] = 'application/msword';

  // Optional. Remove a mime type.
  unset( $mimes['exe'] );

  return $mimes;
}
add_filter( 'upload_mimes', 'my_custom_mime_types' );

Or to allow unfiltered uploads for any type, add to functions:

define( 'ALLOW_UNFILTERED_UPLOADS', true );