Getting Safari to Write Out Correct FileName

Question

I'm building a web app with PHP that uses Excel Writer (XML) for PHP to create an Excel file that the user can download. I've taken a look at the source code for the library, and all it does is echo the generated XML to standard output. Although the file being generated is an XML file, I specifically give it a name with a ".xls" extension so that after it's been downloaded the end-user can double-click it and it will launch Excel on their system and open the file successfully.

This works correctly when downloading the file with Firefox, Chrome and IE, but not with Safari. For example, if I set the name of the file to be 'File.xls', this is the name of the file that gets downloaded. But with Safari the name of the file ends up being 'File.xls.xml'.

The server is running Apache on Mac OS X. I thought that might matter as I'm guessing that the problem has to do with MIME types on the server, but I'm not at all sure. Perhaps I can do something with the link that appears on the page, or perhaps I need to edit a configuration file for Apache? Any help is appreciated.