Rom file type is not supported // own build app

Hi,
I created an app and tested it on two different smartphones, it worked as expected on the first smartphone (an older one, not sure about type and android version) but on the second smartphone "Rom file type is not supported." popped up as I tried to open the .apk, after downloading it per mail without problems.
Second smartphone is a Galaxy 2 android version 4.0.3
The .apk was created using eclipse with the android plugins Android tools -> export signed application package

the manifest:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="..."
package="com.timsapps.phrasendreschmaschine"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="16" />

<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
...
</activity>
...
</application>

</manifest>

settings> application settings> unknown sources is also checked

Why the pop-up, and how can i fix it?

I am a beginner app developer, thank you for your answers/time.

— modified on Dec 10, 2012, 1:26:38 PM

okay I solved it myself, a very easy solution as i expected
newer android smartphones need appinstaller to open the apk files older ones not.