This is my suggestion for the filetype associations in #d.

The AddIn contains two classes:
 - The RegisterFiletypesCommand, that is executed at startup and where the types are registered
 - The RegisterFiletypesOptionPanel, that is shown in the IDE Options.

In the Option panel, you can select the types you want #d to be associated with and whether #d should re-register the types at each startup.
The command, which is autoexecuted at startup, checks if this option is turned on and then performs the task, if necessary.
If this option is turned off, the only point where file types are registered is clicking OK in the option panel.

The file types are listed in the Filetypes.xml file, which has to be located (as well as the icon files) in the subdir "filetypes" of "data\resources".
So types can be dynamically added or deleted. (In case this file is malformed, the addin will load an internal list that is currently the same as in the XML file.)

I don't exactly know how to manage that the option panel pops up the first time #d is started (like the code completion database wizard), but I think that's not a point of extremely much work.

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New Info
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- By default, cmbx and prjx are registered.
- The Addin now automatically unregisters filetypes when they are deselected in the option panel.
- An Unregister.exe is included that takes one parameter: the extension.
- When registering file types, old associations are preserved and restored when unregistering.