mime_types.php

<?php
/**
 *
 */
function new_prefix_pewc_get_permitted_mimes( $permitted_mimes ) {
  $permitted_mimes['pdf'] = "application/pdf";
  $permitted_mimes['svg'] = 'image/svg+xml'; //Adding svg extension
  $permitted_mimes['eps'] = 'image/x-eps'; //Adding photoshop files
  $permitted_mimes['ai'] = 'application/postscript'; //Adding photoshop files
  $permitted_mimes['psd'] = 'image/vnd.adobe.photoshop'; //Adding photoshop files
  $permitted_mimes['xd'] = 'application/zip'; //Adding photoshop files
  $permitted_mimes['ps'] = 'application/postscript'; //Adding photoshop files
  $permitted_mimes['xls'] = 'application/vnd.ms-excel'; // excel file
  $permitted_mimes['xlsx'] = 'application/vnd.openxmlformats-officedocument.spreadsheetml';  // excel file
  $permitted_mimes['csv'] = 'text/csv'; // csv file
  return $permitted_mimes;
}
add_filter( 'pewc_permitted_mimes', 'new_prefix_pewc_get_permitted_mimes' );

function new_prefix_mime_types($mime_types){
  $mime_types['svg'] = 'image/svg+xml'; //Adding svg extension
  $mime_types['eps'] = 'image/x-eps'; //Adding photoshop files
  $mime_types['ai'] = 'application/postscript'; //Adding photoshop files
  $mime_types['psd'] = 'image/vnd.adobe.photoshop'; //Adding photoshop files
  $mime_types['xd'] = 'application/zip'; //Adding photoshop files
  $mime_types['ps'] = 'application/postscript'; //Adding photoshop files
  $mime_types['xls'] = 'application/vnd.ms-excel'; // excel file
  $mime_types['xlsx'] = 'application/vnd.openxmlformats-officedocument.spreadsheetml'; // excel file
  $mime_types['csv'] = 'text/csv'; // csv file
  return $mime_types;
}
add_filter('upload_mimes', 'new_prefix_mime_types', 1, 1 );

/**********************************************************
 * Additionally, you can install the free plugin, WP Extra File Types:
 * https://en-gb.wordpress.org/plugins/wp-extra-file-types/
 * And enable various file types as required
**********************************************************/