How to allow a WebView app to upload a picture from the Gallery of the device on Android Studio?

I am in my first application in WebView. I have a page in my app that allows the user to upload a picture from the gallery of photos of the device to change the avatar.

However, when I test the app, the button doesn't do anything. I tap, tap, tap, double tap ! nothing happens...

I have seen a lot of posts, I tried a lot too, nothing seems to work, some say it is not possible in webview, others say it is not anymore the same method since Lollipop etc... In other words : I AM LOST !

Here is the code of my MainActivity.java :

package com.example.app;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;



public class MainActivity extends Activity {

    private WebView mWebView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mWebView = (WebView) findViewById(R.id.activity_main_webview);

        // Force links and redirects to open in the WebView instead of in a browser
        mWebView.setWebViewClient(new WebViewClient());

        // Enable Javascript
        WebSettings webSettings = mWebView.getSettings();
        webSettings.setJavaScriptEnabled(true);

        // Use remote resource
        mWebView.loadUrl("http://urlofmywebsite");

        // Stop local links and redirects from opening in browser instead of WebView
        //mWebView.setWebViewClient(new MyAppWebViewClient());

        // Use local resource
        // mWebView.loadUrl("file:///android_asset/www/index.html");

    }

    // Prevent the back-button from closing the app
    @Override
    public void onBackPressed() {
        if(mWebView.canGoBack()) {
            mWebView.goBack();
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}