How do I create a default association for files with an unknown extension?

A. Files with an unknown extension will bring up an "open with" dialog when you double click on them however it is possible to associate an application with files of an unknown type.

You can force this using the registry:

  1. Start the registry editor (regedit.exe)
  2. Move to HKEY_CLASSES_ROOT\Unknown\shell
  3. From the Edit menu select New - Key
  4. Enter a name of 'open' and press Enter (don't type the quotes)
  5. Select the new 'open' key
  6. From the Edit menu select New - Key
  7. Enter a name of 'command' and press Enter (don't type the quotes)
  8. Double click the (Default) value
  9. Change to the application used to open, e.g. "NOTEPAD.EXE %1" for the notepad.exe application (you need %1 to pass the document name to notepad and depending on the application you may need %1 %*)
  10. Click OK

If you now double click on a file with no extension it will open with the application selected.

You can optionally delete the HKEY_CLASSES_ROOT\Unknown\shell\openas key and its content but it is not necessary and it will just remove the open with context menu option.

It is possible to just add a notepad option to the context menu of Unknown by using the existing entry in HKEY_CLASSES_ROOT/Unknown/shell, adding a new key value of NotePad and a new Key value of 'open' underneath Notepad then a new Key called 'command' under open. Set the (Default) value of command to Notepad.exe %1. Registry file to perform this:

@="notepad %1"

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