but I'm calling $q->header( -type => 'image/jpeg'); before reading the file data from the file to a buffer variable and then returning that data.

perldoc CGI shows that you have to print the header,

$ perl -MCGI -le"print CGI->header(qw! -type image/jpeg -nph 1!)
HTTP/1.0 200 OK
Server: cmdline
Date: Sun, 08 Mar 2009 08:14:05 GMT
Content-Type: image/jpeg
[download]

What you probably want is from CGI::Application

    # add or replace the 'type' header
    $webapp->header_add( -type => 'image/jpeg', -attachment => 'foo.jp
+g' );
[download]

I had found an article with some examples using a redirect to append a "/image.extension" onto the URL, but that might be old and probably doesn't work anymore (and seems like it was kind of a hack anyway)
It was probably a hack for browsers which ignored attachment headers.

See also overkill anti-caching CGI headers