Add or remove extensions from the ext array by using the wpuf_allowed_extensions filter

add_filter( 'wpuf_allowed_extensions', 'wpufe_custom_upload_extensions');
function wpufe_custom_upload_extensions( $extensions ) {
    $extesions = [
        'images' => ['ext' => 'jpg,jpeg,gif,png,bmp', 'label' => __( 'Images', 'wp-user-frontend' )],
        'audio'  => ['ext' => 'mp3,wav,ogg,wma,mka,m4a,ra,mid,midi', 'label' => __( 'Audio', 'wp-user-frontend' )],
        'video'  => ['ext' => 'mp4,mkv,mov', 'label' => __( 'Videos', 'wp-user-frontend' )], //add or remove extensions from the ext array
        'pdf'    => ['ext' => 'pdf', 'label' => __( 'PDF', 'wp-user-frontend' )],
        'office' => ['ext' => 'doc,ppt,pps,xls,mdb,docx,xlsx,pptx,odt,odp,ods,odg,odc,odb,odf,rtf,txt', 'label' => __( 'Office Documents', 'wp-user-frontend' )],
        'zip'    => ['ext' => 'zip,gz,gzip,rar,7z', 'label' => __( 'Zip Archives', 'wp-user-frontend' )],
        'exe'    => ['ext' => 'exe', 'label' => __( 'Executable Files', 'wp-user-frontend' )],
        'csv'    => ['ext' => 'csv', 'label' => __( 'CSV', 'wp-user-frontend' )],
        'custom'    => ['ext' => 'obj,stl,mtl,blend', 'label' => __( 'Custom Files', 'wp-user-frontend' )],
    ];

return $extensions;
}