How do I get the file contents when an app is chosen to a open file? (Android Kotlin)

I have an Android app written in Kotlin. I want it to open files of a specific format (let's just say ".json" for now). When it is chosen from an app chooser by the user to open a certain file, what do I do next? How do I get that file into my code?

You should to declare a Intent filter.

Add to your AndroidManifest.xml an intent filter to handle the specific file type.. you case a json type.

<activity android:name=".YourActivity">
<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="content" />
    <data android:mimeType="application/json" />
</intent-filter>

And in your Activity code you should to handle these file received, something like that:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_your)

    handleIncomingIntent(intent.data)
} 

private fun handleIncomingIntent(intent: Intent) {
    if (intent.action == Intent.ACTION_VIEW) {
        val uri: Uri? = intent.data
        uri?.let {
            val contents = content resolver.openInputStream(uri)?.bufferedReader()?.readText()
        }
    }
}

Since we are handling files through the content scheme and not directly accessing the file system, there is no need to request READ_EXTERNAL_STORAGE permissions. Content URIs handle file access securely, and your app will not need special permissions for this. Android Permissions

Thanks @CommonsWare for the contributions.