There is a "bug" in Zend_Gdata_Docs with the mimetype.
If you are using a temporary file and the filename as the title it will not auto-magically pull the mimetype for you. It tries to pull the mimetype based on the fileLocation extension which does not exist on a temporary file.
I made a class that works for me, rather than updating the Zend class.
It is called ConvertDoc because I wanted to be able to upload a spreadsheet and download as csv.
What you really need is this...
// get mimetype from original file name
$filenameParts = explode('.', $originalFileName);
$fileExtension = end($filenameParts);
$mimeType = Zend_Gdata_Docs::lookupMimeType($fileExtension);
And pass the $mimetype instead of null.