Regardless of the file type, Cyberduck 3.4 allows to edit files with your preferred editor from the remote server. This allows to edit not only text files but images and other binary content as well with your preferred application directly from within Cyberduck.
The Command-K shortcut and the edit toolbar button will always point to the application set as the default application in the Finder for a given file type. A default editor application can be choosen in the Preferences and set to override the default application that would be used when double clicking the file of a given type in the Finder.