IIS not responding to HTTP request header added in the Rewrite Module

Hi, I'm having problems getting IIS7.5 to accept HTTP range headers, added using the URL Rewrite module v2

Setting up a DASH server and the players based on Flash pass the byte range data via querystring as the Flash player can't send range headers. These players send a request like http://domain.com/test.mp4?bytes=10-50 A .htaccess file is provided that basically takes the 'bytes=10-50' and puts it into a header. I guess other folks are or will be trying to set this up for DASH on Windows servers as well where .htaccess files don't help much. Anyway this is the file.

<IfModule headers_module>
<IfModule rewrite_module>
  RewriteEngine On
  RewriteCond %{QUERY_STRING} ^(.*)bytes=(.*)$
  RewriteRule ^(.*) - [L,E=range:%2]
  RequestHeader set Range "bytes=%{range}e" env=range
</IfModule>
</IfModule>

First problem is that this file won't import into the IIS Rewrite module, which gives the error: cannot be converted to IIS format because of unsupported flags: E

So as the .htaccess wouldn't import I added HTTP_RANGE to the allowed variable list and created a basic test rule that triggers on anything and just sets a HTTP Range header with some fixed values.

<rewrite>
  <rules>
    <rule name="RangeTest">
       <match url="(.*)" />
        <serverVariables>
          <set name="HTTP_RANGE" value="bytes=0-50" />
        </serverVariables>
        <action type="None" />
    </rule>
  </rules>
</rewrite>

But I just can't get it to work, it's like the range header is being ignored. I have a basic .net page that displays all the incoming request headers and I know the rewrite rule is triggering as I can see that the new header with the correct format has been added. I can also modify other headers, e.g. user-agent no problem.

With curl you can see the problem. Rule disabled and sending the range header via curl works fine as you would expect, a 206 partial content response:

>>curl -v --header "Range: bytes=0-40" http://domain.com/test.mp4

> GET /test.mp4 HTTP/1.1
> User-Agent: curl/7.40.0
> Accept: */*
> Range: bytes=0-40
>
< HTTP/1.1 206 Partial Content
< Content-Type: video/mpeg
< Last-Modified: Tue, 10 Mar 2015 18:18:05 GMT
< Accept-Ranges: bytes
< ETag: "bc93df8d5e5bd01:0"
< Server: Microsoft-IIS/7.5
< X-Powered-By: ASP.NET
< Date: Wed, 11 Mar 2015 09:12:07 GMT
< Content-Length: 41
< Content-Range: bytes 0-40/30200
<

Rewrite rule enabled and I get a 200 response, all the content is returned:

>>curl -v http://domain.com/test.mp4

> GET /test.mp4 HTTP/1.1
> User-Agent: curl/7.40.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: video/mpeg
< Last-Modified: Tue, 10 Mar 2015 18:18:05 GMT
< Accept-Ranges: bytes
< ETag: "bc93df8d5e5bd01:0"
< Server: Microsoft-IIS/7.5
< X-Powered-By: ASP.NET
< Date: Wed, 11 Mar 2015 09:15:27 GMT
< Content-Length: 30200
<

So the rule is triggering, it's adding the header and then IIS is not responding to it. It does the same on .jpg files. Am I missing something obvious here, please point me in the right direction.

Thanks, Jon