Since No Tree Punching have ceramic buckets, I think it would be good to allow use them with milk (get milk from cows, craft a cake, drink milk to wear out potion effects)

Did you try:

1. In the config, adding "milk" to the list "CLAY_BUCKET_FLUIDS" and
2. Adding a mod that makes milk a fluid (it is not a fluid in vanilla IIRC, something like Thermal series should do it)
   It should already be a feature.

1. Yes
2. Yes (I'm used Forestry for that)

Hmm, that should be working then. I'll have a look when I get home.

Are you asking if milk buckets exist? Or if they obey the above behavior?

I'm asking about milk bucket existing in case when milk don't registered as fluid and about milk bucket behaviour in both cases

I'm not planning to add another fluid just for this case. I might look into other behavior at some point, presuming another mod adds the respective fluids.