What is the right extension for files with JSONP?

I have a file with JSONP (hard-written callback) and I named it file.jsonp. Unfortunately server treats it as text/plain and browser returns warning

calling it file.json causes the same warning. So what is the right extension? 

Please Help

What I have tried:

json file with extension .json :- File.json  :- it is ok

now jsonp file with extension .jsonp :- File.jsonp  :- is it ok ?

please help