Looking at the HTML isn't likely to help - most servers (as you know from using Apache) base their types on the filenames. You can give hints in HTML to tell the browser what type to expect, but the HTTP headers are the last word.

SourceForge appears to use Apache as its Web server; did you try adding a .htaccess file? Something like the following should do:

AddType application/octet-stream .zImage

(in the same directory as the files).