Double-clicking an Excel XML file opens it up in Word!

We save our Excel and Word files as XML, so that we can do search and replace
operations over the XML text files. This causes the following problem: Even
though the XML files contain information about which MS Office application
was used to create the document (e.g. <?mso-application
progid="Word.Document"?> ), when I double click on an Excel file in Explorer,
the file opens up in Word, regardless of whether it is an Excel XML file or a
Word XML file. Is there a way to make Explorer more intelligent when opening
up such files (e.g. via a registry key setting)?