This error will occur if the browser does not recognize the file as an APK file. Seeing an icon other than the red m-Post icon might also indicate the wrong file extension has been applied.
Do not extract the file. Instead, follow the below instructions for proper installation:
1) Locate the file in your device file manager
2) Tap and hold the file name to bring up a secondary menu
3) Select "Rename"
4) Edit the file name to remove the ".zip" extension
5) Select "Change" on the warning prompt regarding changing the file extension
Once the file name is properly formatted as an APK file, the m-Post icon should appear and you can proceed with the installation