I have been trying to get .arm upload to a website built in codeigniter. I know its not a default mine type so I added it with this:
\'amr\' => \'audio/amr
You could try looking at system/libraries/Upload.php
line 199:
$this->_file_mime_type($_FILES[$field]);
Change that line to
$this->_file_mime_type($_FILES[$field]); var_dump($this->file_type); die();
Now you redo your uploading and see what the actual mime type the browser sent to your server.
One silly and insecure workaround I did was adding that actual mime type to mimes array. But this could change if this question get answered :)