Include file name when sending a binary file

I'm developing a bulletin board which accepts binary submissions via the
form file-upload capability.  When the file is downloaded from the cgi
which sends the requested file, the Content-type is
"application/octet-stream" (is there another better for sending binary
files?)

When this happens, the browser saves the file to disk unless the user
has defined a helper application for this content-type.  But the default
file name is the name of the cgi.  Can a client-side default filename be
specified in the http header?

Jeff Willden