Restrict Certain File Mime Types in WordPress

I recently heard from a client saying their blog was loading very slowly. The issue was caused from a high resolution image in .tiff format which is generally used for print. In order to block these file types add the following plugin to your mu-plugins folder.

Source

<?php
/* prevent uploading of .tif files.
https://wordpress.stackexchange.com/questions/44777/upload-mimes-filter-has-no-effect
 
Full list of mime types
https://codex.wordpress.org/Function_Reference/get_allowed_mime_types
*/

add_filter( 'upload_mimes', 'theme_restrict_mime_types' );
function theme_restrict_mime_types( $mime_types )
{
    $mime_types = array(
        'pdf' => 'application/pdf',
        'doc|docx' => 'application/msword',
        'jpg|jpeg' => 'image/jpeg',
        'gif' => 'image/gif',
        'png' => 'image/png'
    );
    return $mime_types;
}

save as restrict-upload-types.php and upload to your mu-plugins folder.