Newick format

I am trying to generate a tree to be used in Mothur. To accomplish this task, I am using RDP. I am able to download a tree in Newick format from RDP. The suffix of the file is .NWK, and I figured, if I want to use the read.tree command, I have to change .NWK into .TREE. Right? So I tryied to do that but the .NWK remained. How can I make this file compatible for Mothur?
I hope my question is clear

I actually just happened to be successful saving that file as .TREE file. Could you please confirm that I am on the right track?

It shouldn’t matter, so long as you give mothur the correct name. A newick formatted file is a text file as is a tree file. Either should work.