downloading jar file renamed as zip

I have a executable jar file on a webpage that people can download. On some computers the jar-file is renamed as X.zip instead of the real name X.jar when it is downloaded. Why? What can I do to prevent that?
[215 byte] By [Mattias_32a] at [2007-10-1 8:54:42]
# 1
I'm pretty sure that's XP trying to be "helpful" - I've only seen it happen on XP. Thank Micro$oft... I'm afraid i don't know how to stop it, though.
Violettaa at 2007-7-10 0:19:53 > top of Java-index,Desktop,Deploying...
# 2
i m having the same problem.even with a same machine, from some sites, i can download jar files, without being renamed, but from some sites, i got the jars renamed into .zip.the current jre of the machine is 1.4.2, does installing 1.5 help?
erv2a at 2007-7-10 0:19:53 > top of Java-index,Desktop,Deploying...
# 3

Maybe the content-type is different.

As XP recognizes the zip files and can show their contents, and given the method to associate the file type to the name-extension, maybe it renames a dowwload file as "zip" if the content-type is zip, whereas it leaves the unknown jar alone (unrenamed).

BIJ001a at 2007-7-10 0:19:53 > top of Java-index,Desktop,Deploying...