The default application for opening XML files is Internet Explorer. If I simply point to http://www.w3schools.com/xml/note.xml it will open that file. If I download and attach the same file to QC, and subsequently try to open it, it doesnt do anything.
Select a XML file in your file system and right click, select Open With, in the Open with Window, select IE or any other XML viewer or editor, mark 'Always use the selected program to open this kind of file' and click OK.
Now go to QC and try to download or double click the file you want to view.