If you upload APK to a web server and try to download it on mobile be giving the URL of APK, it will fail since web server will block serving of any file type not recognized by it. In order to allow serving of APK file from the IIS web server add following mime-type in the web.config file
The complete entry should be as following
<configuration>
<system.webServer>
<staticContent>
<mimeMap fileExtension=".apk" mimeType="application/vnd.android.package-archive"/>
</staticContent>
</system.webServer>
</configuration>