Many webservers are case-sensitive, but do not have all possible combinations of cases in mapped extensions mapped properly.

By changing the letters in a JSP or a JHTML file extension from lower case to upper case (eg: .jsp or .jhtml becomes .JSP or .JHTML) in a URL the server does not recognize the file extension and sends the file normally. In that manner, a user is able to access the source code to those specific files.

（出处：不详 ）