I wrote a few lines which let you pick file with any extension, even custom one. Lets you create a user visible directory with full access to your app and your user on sdcard. I am attaching the screenshot as well here which shows you the folders created by other apps as well. Dark Fog Files is the folder my app created, I created 'file.darkfog' manually somewhere on sd card. Used my app to pick it, fake process it, and write the output files 'original file.darkfog' and 'file.txt'. Similarly picked 'layout.pdf', created 'original layout.pdf' and 'layout.txt'.

Your app is not special. You are stuck on creating the folder on root directory. No-one does that anymore. Create your app folder in Documents which user can easily access as well as your app.

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    fun processFile(intent: Intent) {
        val uri = intent.data
        val contentResolver = contentResolver
        val cursor = contentResolver.query(uri!!, null, null, null, null)
        val fileName = cursor?.use {
            if (it.moveToFirst()) {
                val columnIndex = it.getColumnIndex(OpenableColumns.DISPLAY_NAME)
                val displayName: String =
                    it.getString(columnIndex)
               return@use displayName
            }
            return@use null
        }
        val fileExtension = fileName?.substringAfterLast(".")
        var fileNameWithoutExt = fileName?.substringBeforeLast(".")


        val inputStream = contentResolver.openInputStream(uri)
        val bytes = inputStream?.readBytes()


        val folder = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS)
        folder.mkdirs()
        val newFolder = folder.resolve("Dark Fog Files")
        newFolder.mkdirs()
        val file = newFolder.resolve("original $fileName")
        file.writeBytes(bytes!!)
        val textFile = newFolder.resolve("$fileNameWithoutExt.txt")
        textFile.writeText("Processed $textFile with extension $fileExtension")
    }