Just resolved an interesting problem today, one of our code breaks because the response header set by the web server did not include Content-Length.
Spent quite a while investigating and turns out this is due to gzip compression. As seen below Content-Encoding is gzip and I think this causes Content-Length to be omitted.
Gzip compression can be disabled on apache via .htaccess config. In my case I disabled all compression to swf file by adding following configuration
<FilesMatch "\.swf$"> SetEnv no-gzip 1 </FilesMatch>