Basically I follow the answer of this thread to implement the file chooser in the upload.
     WebChromeClient mWebChromeClient = new WebChromeClient() {
        @Override
        public void onReceivedTitle(WebView view, String title) {
            if ((title != null) && (title.trim().length() != 0)) {
                setTitle(title);
            }
        }
        public void openFileChooser(ValueCallback<Uri> uploadMsg) {  
         mUploadMessage = uploadMsg;  
         Intent i = new Intent(Intent.ACTION_GET_CONTENT);  
         i.addCategory(Intent.CATEGORY_OPENABLE);  
         i.setType("image/*");  
         MainActivity.this.startActivityForResult(Intent.createChooser(i,"File Chooser"), FILECHOOSER_RESULTCODE);  
        }
       public void openFileChooser(ValueCallback uploadMsg, String acceptType) {
            mUploadMessage = uploadMsg;
            Intent i = new Intent(Intent.ACTION_GET_CONTENT);
            i.addCategory(Intent.CATEGORY_OPENABLE);
            i.setType("image/*");
            MainActivity.this.startActivityForResult(
            Intent.createChooser(i, "File Chooser"),
            FILECHOOSER_RESULTCODE);
        }
     //For Android 4.1
        public void openFileChooser(ValueCallback<Uri> uploadMsg, String acceptType, String capture){
            mUploadMessage = uploadMsg;  
            Intent i = new Intent(Intent.ACTION_GET_CONTENT);  
            i.addCategory(Intent.CATEGORY_OPENABLE);  
            i.setType("image/*");  
            MainActivity.this.startActivityForResult( Intent.createChooser( i, "File Chooser" ), MainActivity.FILECHOOSER_RESULTCODE );
        }
    };
    protected void onActivityResult(int requestCode, int resultCode,  
            Intent intent) 
    {  
        if(requestCode==FILECHOOSER_RESULTCODE)  
        {  
            if (null == mUploadMessage) return;  
            Uri result = intent == null || resultCode != RESULT_OK ? null  
            : intent.getData();
            mUploadMessage.onReceiveValue(result);  
            mUploadMessage = null;  
        }
    }
However, when the result is passed back to the webview, the "filename" of the image are not always exactly its name. (For example, abc.jpg).When using print_r($_FILES["file"]) in my backend PHP, it shows the filename is something like this:

While the file name is not same as the original picture and doesn't get the file extension (.jpg, .gif, etc)inside. But the image itself could be uploaded successfully.
Since I would need to manipulate on the Filename (And most importantly the file extension for type checking) of the image, I would like to ask how to retrieve a image result from the Webview, which would got it's true filename? (And its file extension?)
 
    