The 'attachment' macro parameter type does not work in User Macros

Raised from a CAC comment.

You can specify that a plugin macro has an 'attachment' parameter type. For an example see the multimedia macro.

If you try to use 'attachment' parameters in a User Macro then the Macro Browser will render the select box but it will not populate with parameters (or even shown the 'no parameters found message').

To be fair, we haven't documented this type as being available to User Macros but it is documented for plugin macros. It should work for User Macros as well.