File name and hook_nodeapi

Hey,
I have a custom module on my site using hook_nodeapi which basically updates another table with the path to the file uploaded when a node is created or updated.

I'm using: $node->field_image['0']['filepath'] to get the path. But although I'm able to get the right path for the file, I am unable to get the updated file name.

for example:
file uploaded by user: user_picture_of_London.jpg
path settings: pictures/cities
rename settings: [uid]-[cityid].[fileExtension]

instead of getting something like: "pictures/cities/1-21.jpg"
I get: "pictures/cities/user_picture_of_London.jpg"

I can SORT OF overcome that by reconstructing the token in my module and recreate the file name but then I have a problem with getting the right file extension (could be jpg, gif, png etc..)

Is there anyway to overcome this problem in a more efficient way?

Thanks for the help, and thank for this great module!