CodeIgniter: "The filetype you are attempting to upload is not allowed."

I'm experiencing a very odd upload problem. Here's the relevant view file:

<form action="http://localhost/index.php/temp/upload/" method="post" enctype="multipart/form-data"><fieldset><input type="file" name="userfile"/><input type="submit" value="Upload"/></fieldset></form>

And here's my temp controller's upload() method:

public function upload(){    $config['upload_path']   = FCPATH . 'uploads' . DIRECTORY_SEPARATOR;    assert(file_exists($config['upload_path']) === TRUE);    $config['allowed_types'] = 'avi|mpg|mpeg|wmv|jpg';    $config['max_size']      = '0';    $this->load->library('upload', $config);    if ($this->upload->do_upload('userfile') === FALSE)    {        // Some error occured        var_dump($this->upload->display_errors('', ''));        var_dump($_FILES);    }    else    {        // Upload successful        var_dump($this->upload->data());    }}

When I upload an AVI video, everything works fine. When I upload, say, a WMV video, I get the following var dumps:

string 'The filetype you are attempting to upload is not allowed.' (length=57)array'userfile' =>     array'name' => string 'wmv.wmv' (length=7)'type' => string 'video/x-ms-wmv' (length=14)'tmp_name' => string 'C:\wamp\tmp\php2333.tmp' (length=23)'error' => int 0'size' => int 83914

The "wmv" extension is being interpreted as the MIME type: video/x-ms-wmv. This should be fine since my config/mimes.php has the following:

'wmv' =>  array('video/x-ms-wmv', 'audio/x-ms-wmv')

It's a similar situation when I try uploading other files. So far, the only one that seems to work is my test AVI video.

Any ideas what might be wrong?

UPDATE 1:

One my machine, only AVI uploads. On another developer's machine, no files upload. On yet another developer's machine, all supported files upload. Are these browser or server issues?