Opening a .apk in Android shows a file list instead of installing

[Android Central Question]

For a client I have build a private app, just a few devices use it. The user can download the .apk by navigating to a certain URL on their device. A few weeks ago everything was working great, but since a week the .apk shows a file list instead of installing the app.

Does anyone has any idea wat the cause of this problem may be?

 

[Rukbat]

The customer is opening the .apk file in the wrong app - it should be opening the default installer (whichever one the particular device has set as default), not a .zip extractor (which is what appears to be happening - since an .apk file is just a .zip file with a different extension).