Open any type of file

FileUtils.java

public class FileUtils {

    /**
     * @return The MIME type for the given file.
     */
    public static String getMimeType(File file) {

        String extension = getExtension(file.getName()).toLowerCase();

        if (!TextUtils.isEmpty(extension) && extension.length() > 1)
            return MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension.substring(1));

        return "application/octet-stream";
    }


    /**
     * Gets the extension of a file name, like ".png" or ".jpg".
     *
     * @return Extension including the dot("."); "" if there is no extension or fileName is null.
     */
    @NonNull
    public static String getExtension(String fileName) {
        if (fileName == null) {
            return "";
        }

        int dot = fileName.lastIndexOf(".");
        if (dot >= 0) {
            return fileName.substring(dot);
        } else {
            // No extension.
            return "";
        }
    }
  
}

OpenFile.kt

    override fun openFile(path: String) {
        val newIntent = Intent(Intent.ACTION_VIEW)
        val file = File(path)
        val mimeType = FileUtils.getMimeType(file)

        if (Build.VERSION.SDK_INT < 24) {

            newIntent.setDataAndType(Uri.fromFile(file), mimeType)

        } else newIntent.setDataAndType(Storage.getUriFromFile(file), mimeType)

        newIntent.flags = Intent.FLAG_ACTIVITY_NEW_TASK
        if (Build.VERSION.SDK_INT >= 24) {
            newIntent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
        }

        if (newIntent.resolveActivity(requireContext().packageManager) != null) activity!!.startActivity(newIntent)
        else Toast.makeText(activity, R.string.no_handler, Toast.LENGTH_LONG).show()

    }

Returns the extension of file (not including the dot), or an empty string if it doesn't have one
val fileExtension : ((path: String) -> String) = { File(it).extension }
Returns the extension of file (including the dot), or an empty string if it doesn't have one
val fileExtensionWithDot: ((path: String) -> (String)) = { File(it.extension).let { ext -> if (ext.isEmpty() "" else ".$ext") } }

There is no "extension" method in java.io.File package in android SDK 28.