IIS6 ASP.NET - content-length header not set for EXE downloads

Question

When downloading a file from an IIS6-hosted ASP.NET MVC 3 application, the content-length header is not set for .exe files.

MIME-types appear to be correctly set. ( .exe is application/octet-stream ). Static content compression is disabled.

As far as I can tell, this problem is specific to .exe files. .zip works fine.

Suggestions?

Update

Here are the headers that are being sent:

Accept-Ranges:bytes
Cache-Control:max-age=86400
Compression-Control:whitespace
Content-Encoding:gzip
Content-Location:http://...../someFile.exe
Content-Type:application/octet-stream
Date:Wed, 28 Aug 2013 15:45:52 GMT
ETag:"5397aeeb6e4ace1:0"
Last-Modified:Mon, 06 May 2013 15:32:30 GMT
Transfer-Encoding:chunked
Vary:Accept-Encoding

Answer 1

Turned out we had an ISAPI filter running that was stripping out some headers, including this one.

Disabling the filter fixed the problem.