HI all
I have a design customarization. In that I have the following
Customized
Type Design Area
nested content contact us
When I click on the 'contact us', design area and 'acquire the lock' and then scrolled down where it says 'Nested Asset' and click on the change button the left handside panel turns pink which is all fine. But when I try to select a 'file asset' of 'contact us' it doesn't allow it? Is this normal?
Cheers 
Nested content design areas allow you to specify which types of assets can be nested, like this:
Check to make sure your "type_codes" are set appropriately.
Daniel, when I tried value="page file" and or "text_file" (or a number of other combinations) I still couldn't get it to recognise a text file.
*puzzlement*
I would have thought the type codes would be those displayed in the asset tree eg: "Text File (text_file)"
aha! found it all by meself 
I have to delete the customisation involved and then recreate it - THEN it recognises text files
interesting… more playing ensues.