to handle file types not permitted for security reasons

mime-extension-exception.php

<?php 
function add_allow_upload_extension_exception( $types, $file, $filename, $mimes ) {
  // Do basic extension validation and MIME mapping
	$wp_filetype = wp_check_filetype( $filename, $mimes );
	$ext         = $wp_filetype['ext'];
	$type        = $wp_filetype['type'];
  if( in_array( $ext, array( 'ai', 'eps' ) ) ) { // if follows illustrator files have
    $types['ext'] = $ext;
    $types['type'] = $type;
  }
  return $types;
}
add_filter( 'wp_check_filetype_and_ext', 'add_allow_upload_extension_exception', 99, 4 );