"Open in External Viewer"

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There's no way to do this now. The only option is to bind the command open-ext-selected-from-project to a key binding using the User Interface > Keyboard > Custom Key Bindings preference and then use that. Or add a toolbar item for this command (right-click on the toolbar to configure it).

You can set the command line to use for opening something externally by mime type from the Files > External Display > File Display Commands preference, which may allow you to set things up to reuse an existing designer if the command line they provide supports that. The mime type is what Python's mimetypes module guesses for the extension:

  import mimetypes
  print(mimetypes.guess_type('x.png'))

Or perhaps easier is to use a mime type of * in the preference and write an opener script that does the right thing based on what the file is.