How can I allow more file extensions with drupal file uploads?

I've got a module that has to let users upload files and everything works as long as the files are in the standard array of allowed extensions. I've tried using file_validate_开发者_运维技巧extensions, but this doesn't seem to change anything.

This is the code I'm using to upload now (the docx extension is added to the standard drupal allowed ones, but it doesn't seem to get picked up):

$fid = $form_state['values']['attachment'];

$file = file_load($fid);
if($file != null){
    file_validate_extensions($file, "jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp docx");
    $file->status = FILE_STATUS_PERMANENT;
    file_save($file);
}

I just looked to this Drupal API, and it seems that you can use the function "file_save_upload" (with $validator as an array of valid extension), this get the file in a temporary state. And then, you have to call "file_save" to make it permanent.