velopack/docs/hosting-releases-on-iis.md

Hosting on IIS

All versions of IIS (including Microsoft Azure PaaS) deny serving files when the extension MIME type is unknown. If you are hosting your updates in this manner then you will need to add a Web.config to your downloads repository as follows:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <staticContent>
      <mimeMap fileExtension=".nupkg" mimeType="application/zip" />
      <mimeMap fileExtension="." mimeType="text/plain" />
    </staticContent>
  </system.webServer>
</configuration>

eg: ~/downloads/Web.config