Custom filefield tokens in my own module?

Hi there,

I want to create a custom file directory token from the first two letters of the filename,
i.e. /w/e/welcome-gif

If I add tokens to filefield.token.inc, it works fine. Something like this:
$tokens['filefield-a'] = substr(check_plain($item['filename'], 0 ,1);
$tokens['filefield-b'] = substr(check_plain($item['filename'], 1 ,1);

However, I want to put this in my own module. I'm following the tokenstarter module. It's all very easy to do,
BUT, my tokens aren't recognized.

function my_tokens_token_list($type = 'all') {
The $type here is always "user" or "node", but never "field" or "all". It appears my custom tokens aren't available to filefields, they don't show up.

So what's the trick here? How to create custom filefield tokens? Why does the same code work in filefield.token.inc, but not in a custom module?