​OK, I put some time into this and here is what I have found.

I set up a project directory with two subdirectories called src and templates.

Then I created a .tm_properties file in the project directory that contains:
[ src/** ]
fileType  = 'source.ruby'

[ templates/** ]
fileType  = 'text.html.basic'

1. When I select a directory in the file browser and the selected directory is not open (ie. the triangle is pointing to the right) and press ^⌘N the file is created not in the selected directory but as a sibling to the selected directory.  Perhaps this is by design?  So it does not get the assigned type because it is in a different directory.  If I configure a type for the parent directory it correctly gets assigned.  Good.

2.  When I select a directory in the file browser and the selected directory is open (triangle points down) and press ^⌘N, the file is created in the selected directory and is assigned the correct type. Perfect.

3. When I select a directory in the file browser and press ⌥⌘N, (regardless of whether the directory is open or closed) I always get an untitled document with type 'text.plain'.   When I subsequently save the file it correctly goes into the directory that was selected.

- Seems inconsistent that the open/close state of the directory affects one operation but not the other.

- I'm guessing that with ⌥⌘N the file type is not being assigned because the untitled doc does not yet actually

exist in the directory.  But since TextMate knows where it will be saved, perhaps the logic could modified so that this works?

- For now the workaround is to make sure the directory is open in the file browser and use ^⌘N.

Hope this helps.