Please Change the MIME-TYPE mapping of apk to application/vnd.android.package-archive
How to add a MIME type to a Web site or application
1. Open Internet Information Services (IIS) Manager:
o If you are using Windows Server 2008 or Windows Server 2008 R2:
§ On the taskbar, click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
o If you are using Windows Vista or Windows 7:
§ On the taskbar, click Start, and then click Control Panel.
§ Double-click Administrative Tools, and then double-click Internet Information Services (IIS) Manager.
2. In the Connections pane, go to the site, application, or directory for which you want to add a MIME type.
3. In the Home pane, double-click MIME Types.
In the MIME Types pane, click Add... in the Actions pane.
In the Add MIME Type dialog box, add the file name extension as .apk and MIME type as application/vnd.android.package-archive, and then click OK.