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

[eluser]Unknown[/eluser]
The problem is with mime type only, you can trace your flow in the file
system\libraries\Upload.php ---- public function is_allowed_filetype($ignore_mime = FALSE)
the only way this function return a true value is if

1. $this->file_type exactly matches $mime.
2. $this->file_type is present in the array $mime.

you can check this way where does the problem lies.

One strange observation, same extension file(.doc) returns different mime types on different servers.