Creating app which opens a custom file extension

I think you need to do that type of customization via intent-filter something like:

<intent-filter android:icon="your_drawable-resource"
               android:label="your_string_resource"
               android:priority="integer"> 
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <data android:scheme="file" />
    <data android:host="*" />
    <data android:pathPattern=".*\\.YOUR_CUSTOM_FILE_EXTENSION" />
</intent-filter>

Also you should look:

• Custom Filetype in Android not working
• Android intent filter for a particular file extension?
• android intent filter for custom file extension

One possible answer is shown here . Try some customisation for intent filters.

<intent-filter android:priority="999">
    <action android:name="android.intent.action.VIEW" />

    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.BROWSABLE" />
    <category android:name="android.intent.category.OPENABLE" />

    <data android:host="*" />
    <data android:mimeType="application/octet-stream" />
    <data android:pathPattern=".*\\..*\\..*\\..*\\..*\\.yourextension" />
    <data android:pathPattern=".*\\..*\\..*\\..*\\.yourextension" />
    <data android:pathPattern=".*\\..*\\..*\\.yourextension" />
    <data android:pathPattern=".*\\..*\\.yourextension" />
    <data android:pathPattern=".*\\.yourextension" />
    <data android:scheme="content" />
</intent-filter>