Can't upload .xls

[eluser]frist44[/eluser]
If I set

Code:

$config['allowed_types'] = 'doc'

I can upload a .doc file just fine. However, if I set

Code:

$config['allowed_types'] = 'xls';

and upload an .xls, I get an error saying "The filetype you are attempting to upload is not allowed."

Any ideas why .xls is behaving differently?