How to tell what program will open a particular file extension


This tip shows how to use the assoc and ftype command in Microsoft Windows to figure out which program will open a particular file.

These commands are built into Microsoft Windows XP. No additional software is required.

It also shows the detailed command-line options for the program.


Command Step-by-Step Explanation
Open a command prompt
  1. From the Start Menu, choose Run...
  2. Type CMD
  3. Press OK
These are command-line programs that must be run from a command prompt.
Run the ASSOC command
  1. Type assoc .ext (where .ext is the extension you are interested in)
  2. You should see something like .ext=MyExtFile. You will use the MyExtFile value in the next step.
This displays the "Association" for this extension. There isn't much you can do with this except the next step.
Run the FTYPE command
  1. Type ftype MyExtFile (where MyExtFile is the value obtained in the previous step)
This shows the command line that will be used to open files with this association.

LATER: if it shows CLSID, interpreting the % stuff in the command line