[Wordpress issuses] win7guru on "Why can't I import my Posterous wordpress_export_1.xml file?"

I am having trouble importing my posts from Posterous backup. I try to import the wordpress_export_1.xml file (which is smaller than 15MB), but I get an error as follows

Import Posterous

Sorry, there has been an error
The file is not an xml file. Please see instructions for downloading and using the Posterous backup file.

I really would not like to have to repost every single post manually. Please help me figure this out.

The blog I need help with is win7guru.wordpress.com.

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