Most CGI programmers are familiar with Content-Type. Sections 3.7, 7.2.1, and 14.17 of the HTTP specification cover the details. mod_perl has a content_type( ) method to deal with this header:

$r->content_type("image/png");

Content-Type should be included in every set of headers, according to the standard, and Apache will generate one if your code doesn't. It will be whatever is specified in the relevant DefaultType configuration directive, or text/plain if none is active.