I was having the same problem when I received a html to put inside my project and rename it to .jsp. To solve the problem, I needed to what people above already said, that is, to change text encoding in Eclipse Preferences. However, before renaming the files to .jsp, it was necessary to include the following line in the beginning of each .html file:
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
I believe this forced Eclipse to understand that it was necessary to change file encoding when I tried to rename .html to .jsp.