Playing with Extensions (How ?)

What it is a file extension?

A file extension tells the computer and user what kind of information is stored within a file.

The following figure will give you a general idea of file extensions:

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How File Extensions Work

Let's see what role extensions play when we open a file.

• When user double clicks on a file Operating System searches for HKCR\ <.ext> key
• If OS finds extension is not registered then user is redirected to open with dialog
• If extension is registered then OS searches for the PROG ID.
• If PROG ID is available OS searches for the shell command in PROG ID
• If PROG ID is not available then OS searches shell command in HKCR\ <.ext>
• If shell command is available then file is opened using shell command
• If shell command is not available the user is redirected to open with dialog

Following Workflow will give the better idea:

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Following figure shows PROG-ID and Shell command:

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Now here;

• Files types are registered as file Extensions.
• Extensions are associated with a ProgID.
• The ProgID specifies the shell commands.

Let's see what the shell commands are...

Shell Commands are nothing but the Verbs defined in the Verb Table

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\<ProgID>\Shell\

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\<ProgID>\Shell\<Verb>\
		
		@="Display Name"

		Command\@ = "<Path to EXE> <Parameters>"
		Command\Command = "<WI Encrypted Value>"
		
		ddeexec\@ = "DDE Command"
		ddeexec\Application\@ = "App Name"
		ddeexec\Topic\@ = "Topic Name"

Scenario:

We have to create one extension .wrd which would open file with .wrd extension in Word

• Create a key under HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.wrd
  
  

  Click to view.

• @ = Word.Document.8
  
  

  Click to view.

So this is how it works!