Can you try the following?
Save the following text as xml_auto_file.reg:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Classes\xml_auto_file]
[HKEY_CURRENT_USER\Software\Classes\xml_auto_file\shell]
[HKEY_CURRENT_USER\Software\Classes\xml_auto_file\shell\open]
[HKEY_CURRENT_USER\Software\Classes\xml_auto_file\shell\open\command]
@="\"C:\\Program Files (x86)\\Notepad++\\notepad++.exe\" \"%1\""
[HKEY_CURRENT_USER\Software\Classes\.xml]
[HKEY_CURRENT_USER\Software\Classes\.xml\OpenWithProgids]
"xml_auto_file"=""
If your Notepad++ installation directory differs from C:\Program Files (x86)\Notepad++ you will need to modify this accordingly.
Import this registry with regedit.exe and try again.