Describe the bug
Despite being able to put water and lava into the fluid tank upgrade, it doesn't accept milk. This seems like a bug, if not, sorry for mislabling a feature request.

To Reproduce
Steps to reproduce the behavior:

1. Open backpack
2. Insert fluid tank upgrade
3. Try to empty milk bucket into tank
4. It's not emptied

Expected behavior
I expected to be able to empty milk into the fluid tank since it's a fluid.

Screenshots

Versions
Backpacks v 3.20.1.1012
Forge 47.2.0

Is milk a fluid in your modpack? Meaning is there some other mod in your modpack that add milk fluid? Sophisticated definitely doesn't add that.

Hmm. I'm not a huge modded player so I'm not sure if I understand your question correctly.
Would I need a mod that somehow adds milk as a fluid? I would have understood the fluid tank so it allows me to trade a few storage slots for 20 buckets of - well, anything that exists in a bucket, like milk.
But I'm starting to suspect that I'm misunderstanding the way the fluid tank works quite drastically!

Edit, I think I understood the questions, yes, it is, according to JEI:

In vanilla, milk isn't a fluid - it doesn't exist as a liquid block or anything, just the bucket item. Honey is the same way. So there isn't a definition for a liquid that can be added to a tank. Forge does actually add a milk fluid, but it's only enabled if a mod requests it to ensure the base install doesn't change gameplay. Maybe Sophisticated should call enableMilkFluid()?

If you have another mod that's done that (try placing milk in-world), it should work then.

Ok, I tested more. Since I had milk as a fluid according to JEI (I suppose through create, because I don't think any of the few other mods I have would add that) I tried various things.

I had the fluid upgrade in a copper backpack. Putting a bucket of milk into the input slot did nothing.

I now put the fluid tank module into my gold backpack. Putting a bucket of milk into the fluid tank input slot filled the fluid tank and emptied the bucket!

I then realized I can also right click the bucket into the fluid area to empty and fill. That works both in the copper and the gold backpack!

EDIT, I upgraded the copper backpack to an iron backpack and it works like expected in the gold backpack.

EDIT2, I tested with a copper tank in creative and I can add milk into the fluid tank by dropping a bucket in the input slot. I can't recreate the issue, but it's behaving like expected now. Thanks y'all for the help!

Just tested and even with copper backpack and tank in it if milk is registered as fluid I can get bucket drained into it. So I would need steps how to recreate to be able to do anything here.

Thanks, closing ...

Yeah, as I said, I can't recreate it anymore myself. It definitely didn't work with the first copper backpack, but any other copper backpack I now tested with, it works fine. I don't think it's worth trying to investigate this any further, so thanks for your help, I really appreciate it, feel free to close this as solved!