With WAR-packaged webapp, getServletContext().getResource("/mydata/../mydata/myfile.txt") should return the same URL as getServletContext().getResource("/mydata/myfile.txt") but returns NULL instead. It's ok for folder-based webapp. It was working in Tomcat 6.0.20. It looks like Bug 10629 (WARDirContext.java regression?).
Thanks for the report. This has been fixed in trunk and 7.0.x and will be included in 7.0.23 onwards.