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I had taken over an older web project where some files had the extension ".jsp" but were really XHTML files. So I wanted to change this and change their extension to ".xhtml". But this is not possible in NetBeans. Therefore could you add a check-box "Show file extension" that would allow to show and thus also to change the extension of a file. (Please reassign if this was not the right place to submit this issue.)
I hope that I assigned it to proper category. "host" most probably will be closed as wontfix since there is a way how to change the file extension. Invoke on the file Properties and you are able to change the extension there.
Rename for non-java files is handled by the default rename-action. Reassigning to platform/actions.
Change the extension in property sheet.