Hello Fil,
If you scale the model and then select the entire model for fillet, it will succeed.

Open model and select and run scale. Type 0 and hit enter, then type .1 to scale it down by 10.. Then select the entire solid and run fillet with a value of .2 rescale back by 10.

Note that the fillet actually does these areas here too. I think this is the area that is giving the fillet issues with other methods...



I've attached your model here with the fillet from the above method.