I would imagine it is because IIS doesn't serve unknown file extensions. You need to add the MIME type to the overall IIS settings, or the web.config. As you asked for in the comment, I'd probably recommend placing it in the web.config since it doesn't require additional configuration from a sysadmin and everything is in one place.
<configuration>
<system.webServer>
<staticContent>
<mimeMap fileExtension=".iso" mimeType="application/octet-stream" />
</staticContent>
</system.webServer>
</configuration>