How do I set a default app for an extension that is not listed?

Configure Windows File Association Pointers

1. Open up an elevated [cmd.exe] command prompt as administrator
2. Type in ASSOC .xml=xmlfile and press Enter

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Further Resources

• Windows Environment Variables

• ASSOC /?

  Displays or modifies file extension associations
  
  ASSOC [.ext[=[fileType]]]
  
    .ext      Specifies the file extension to associate the file type with
    fileType  Specifies the file type to associate with the file extension
  
  Type ASSOC without parameters to display the current file associations.
  If ASSOC is invoked with just a file extension, it displays the current
  file association for that file extension.  Specify nothing for the file
  type and the command will delete the association for the file extension.
  

• FTYPE /?

  Displays or modifies file types used in file extension associations
  
  FTYPE [fileType[=[openCommandString]]]
  
    fileType  Specifies the file type to examine or change
    openCommandString Specifies the open command to use when launching files
                      of this type.
  
  Type FTYPE without parameters to display the current file types that
  have open command strings defined.  FTYPE is invoked with just a file
  type, it displays the current open command string for that file type.
  Specify nothing for the open command string and the FTYPE command will
  delete the open command string for the file type.  Within an open
  command string %0 or %1 are substituted with the file name being
  launched through the assocation.  %* gets all the parameters and %2
  gets the 1st parameter, %3 the second, etc.  %~n gets all the remaining
  parameters starting with the nth parameter, where n may be between 2 and 9,
  inclusive.  For example:
  
      ASSOC .pl=PerlScript
      FTYPE PerlScript=perl.exe %1 %*
  
  would allow you to invoke a Perl script as follows:
  
      script.pl 1 2 3
  
  If you want to eliminate the need to type the extensions, then do the
  following:
  
      set PATHEXT=.pl;%PATHEXT%
  
  and the script could be invoked as follows:
  
      script 1 2 3