I don't see anything in your code that would care about what extension your files have. The only way you could get null in zipFile is if there is an exception thrown. I suggest printing out a stack trace in your catch block. It's generally not a good idea to have empty catch blocks. Either handle the exception or log it in some way (or both).
Also, I suggest using "/" as the file separator. It works on every operating system I've tried (including Windows), and has the added advantage of not being so ugly. To be truly cross-platform, you really should query the system properties for the file separator, but in practice "/" always works anyway.