Add menu item to windows context menu only for specific filetype

There's another key on the registry that works independently of user's default programs: HKEY_CLASSES_ROOT\SystemFileAssociations. Since nobody mentioned it on this question... No need to check ProgID before adding the context menu item. Example:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\SystemFileAssociations\.mkv\shell\subtitle]
@="Search subtitles..."

[HKEY_CLASSES_ROOT\SystemFileAssociations\.mkv\shell\subtitle\command]
@="\"D:\\Tools\\subsearch.exe\" \"%1\""

Reference: https://docs.microsoft.com/en-us/windows/desktop/shell/app-registration#registering-verbs-and-other-file-association-information

Additional Considerations:

The HKEY_CLASSES_ROOT subtree can be written to but in general is a view formed by merging

• HKEY_CURRENT_USER\Software\Classes
  • file type registration visible to the current user only
• HKEY_LOCAL_MACHINE\Software\Classes
  • globally register a file type on a particular computer

You can register to those classes instead/aswell

The (ProgID) defined verbs have priority over the same ones defined in ...\SystemFileAssociations\ , but are dependent on that particular Application, When that application uninstalls, it would normally delete its registry entry, along with what modifications/additions you may have done under that key. Or if the default (ProgID) is changed, your modifications will no longer be in effect.

The ...\SystemFileAssociations\ registrations are stable even when users change/uninstall the default programs.

1. Identify the file type (ProgID) for .jpg files

   This can be done by checking the default value of HKEY_CLASSES_ROOT\.jpg. It could be anything based on what you've installed, but for the purposes of this example, we'll call it jpegfile, a common default.

2. Set the context menu item (verb) properties for that file type

   You can set per-user context menu items in HKEY_CURRENT_USER\Software\Classes\jpegfile\shell. This key has a list of verbs for the file type. There is a similar key in HKEY_LOCAL_MACHINE\Software\Classes\jpegfile\shell, and these are the system defaults for the file type. You can put a verb key there too, but if the same key exists in HKCU, it will be overridden, so be advised.

3. Set the command value

   The bare minimum key value that needs to be set to get it to work is the default value of the command subkey. You need to set that with the path to your application, like so: HKEY_CURRENT_USER\Software\Classes\jpegfile\shell\open_with_myapp\command would be set to "c:\path\to\myapp.exe" "%1". Now a context menu for .jpg files will have a "open_with_myapp" item which will launch your app when clicked, and pass the file name of the selected file as a parameter. Of course, how your application processes parameters is up to you, so you'd need to set the parameter string to something your app can process.

4. Set other verb properties

   I'd imagine you're probably going to want the context menu item to read something a little more friendly than the key name. You can have the context menu display whatever label you want for your item by setting the default value of that key (open_with_myapp).

That's your basic overview. Definitely check out my answer to this question about associating a file, which has a similar answer:

• Create registry entry to associate file extension with application in C++