Browser Downloading ASP App Instead of Displaying

Question

Windows Server 2012 R2 / IIS 8.5

In IIS > Sites > Default Website I have an (ASP) application, when I visit the localhost URL a file is downloaded instead of the page being displayed.

The files are located at C:\inetpub\wwwroot\rooms and within this folder there are various .dll files, folders and a web.config. The contents of the web.config are;

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <directoryBrowse enabled="false" />
        <handlers accessPolicy="Read, Execute, Script">
            <remove name="ISAPI-dll" />
            <add name="ISAPI-dll" path="*.dll" verb="*" modules="IsapiModule" scriptProcessor="C:\inetpub\wwwroot\rooms\rooms.dll" resourceType="File" requireAccess="Execute" allowPathInfo="true" preCondition="bitness32" />
        </handlers>
        <defaultDocument enabled="true">
            <files>
                <add value="rooms.dll" />
            </files>
        </defaultDocument>
    </system.webServer>
</configuration>

I have set the Default Document in IIS to the relevant .dll file, this is what's being downloaded when I visit localhost/rooms

I have restarted the IIS server and the site itself a few times. There are other sites hosted on the same server and they are working as expected and accessible via a similar URL. I've also tried various browsers.

I have checked windows features and roles and can confirm that ASP.NET 3.5 and ASP.NET 4.5 are displaying as (Installed), as is the ISAPI Extensions and ISAPI Filters.

I have very little experience with IIS or ASP.

• Why is the application being downloaded rather than displayed?
• How can I begin to troubleshoot this?