Opening email attachments in Android

Question

I try to open a specific email attachment type with my activity, the attachment itself is a plain text windows style ini file with the extension *.os. In the email this file is attached as "application/octet-stream". I tried the following intent-filter to catch the attachment with my activity:

         <intent-filter>
            <action android:name="android.intent.action.VIEW"/>
            <category android:name="android.intent.category.DEFAULT"/>
            <category android:name="android.intent.category.BROWSABLE"/>
            <data android:scheme="http" android:host="*" android:pathPattern=".*\\.os"/>
            <data android:scheme="https" android:host="*" android:pathPattern=".*\\.os"/>
            <data android:scheme="content" android:host="*" android:pathPattern=".*\\.os"/>
            <data android:scheme="file" android:host="*" android:pathPattern=".*\\.os"/>
        </intent-filter>

The Email app is fetching the attachment but is then telling me "The attachment cannot be displayed". What can I do? How can I debug this?