在奇巧，你可以使用存储访问框架。

存储访问框架/编写客户端应用程序

谷歌自己的浏览器为这个问题提供了一个全面的解决方案，它保证了自己的类:

openFileChooser在Android 4.0.4中的实现

android4.0.4中的UploadHandler类

自定义WebChromeClient:

class AppChromeClient(private val fragmentWeakReference: WeakReference<WebViewFragment>) :
WebChromeClient() {
private var openFileCallback: ValueCallback<Array<Uri>>? = null

override fun onShowFileChooser(
    webView: WebView?,
    filePathCallback: ValueCallback<Array<Uri>>?,
    fileChooserParams: FileChooserParams?
): Boolean {
    if (filePathCallback == null) {
        return (super.onShowFileChooser(webView, filePathCallback, fileChooserParams))
    }
    openFileCallback = filePathCallback
    val webViewFragment = fragmentWeakReference.get() ?: return false
    webViewFragment.launchGetMultipleContents("*/*")

    return true
}

fun receiveFileCallback(result: Array<Uri>) {
    openFileCallback?.onReceiveValue(result)
    openFileCallback = null
}
}

WebViewFragment:

class WebViewFragment : Fragment() {
    private var _binding: FragmentWebviewBinding? = null
    private val binding get() = _binding!!
    private lateinit var webView: WebView
    private val chromeClient = AppChromeClient(WeakReference(this))
    private var contentLauncher: ActivityResultLauncher<String> = getMultipleContentLauncher()

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        _binding = FragmentWebviewBinding.inflate(inflater, container, false)

        webView.webChromeClient = chromeClient
        val url = requireContext().getString(R.string.app_domain)
        webView.setting.javaScriptEnabled = true
        webView.loadUrl(url)
        return binding.root
    }


    private fun getMultipleContentLauncher(): ActivityResultLauncher<String> {
        return this.registerForActivityResult(ActivityResultContracts.GetMultipleContents()) { list ->
            if (list.isEmpty()) {
                showToast("No files selected")
            }
            chromeClient.receiveFileCallback(list.toTypedArray())
        }
    }


    fun launchGetMultipleContents(type: String) {
        contentLauncher.launch(type)
    }
}

在棒棒糖5.0中，谷歌添加了一个官方方法WebChromeClient.onShowFileChooser。它们甚至提供了一种自动生成文件选择器意图的方法，以便它使用输入接受mime类型。

public class MyWebChromeClient extends WebChromeClient {
        // reference to activity instance. May be unnecessary if your web chrome client is member class.
    private MyActivity activity;

    public boolean onShowFileChooser(WebView webView, ValueCallback<Uri[]> filePathCallback, FileChooserParams fileChooserParams) {
        // make sure there is no existing message
        if (myActivity.uploadMessage != null) {
            myActivity.uploadMessage.onReceiveValue(null);
            myActivity.uploadMessage = null;
        }

        myActivity.uploadMessage = filePathCallback;

        Intent intent = fileChooserParams.createIntent();
        try {
            myActivity.startActivityForResult(intent, MyActivity.REQUEST_SELECT_FILE);
        } catch (ActivityNotFoundException e) {
            myActivity.uploadMessage = null;
            Toast.makeText(myActivity, "Cannot open file chooser", Toast.LENGTH_LONG).show();
            return false;
        }

        return true;
    }
}


public class MyActivity extends ... {
    public static final int REQUEST_SELECT_FILE = 100;
    public ValueCallback<Uri[]> uploadMessage;

    protected void onActivityResult(int requestCode, int resultCode, Intent data){
        if (requestCode == REQUEST_SELECT_FILE) {
                if (uploadMessage == null) return;
                uploadMessage.onReceiveValue(WebChromeClient.FileChooserParams.parseResult(resultCode, data));
                uploadMessage = null;
            }
        }
    }
}

对于KitKat之前的Android版本，其他答案中提到的私有方法是有效的。我还没有为KitKat(4.4)找到一个好的解决方案。

我找到了一个适合我的解决方案!在proguard-android.txt文件中增加一条规则:

-keepclassmembers class * extends android.webkit.WebChromeClient {
     public void openFileChooser(...);
}

这个解决方案也适用于蜂窝和冰淇淋三明治。似乎谷歌引入了一个很酷的新特性(accept属性)，并且忘记实现向后兼容性的重载。

protected class CustomWebChromeClient extends WebChromeClient
{
    // For Android 3.0+
    public void openFileChooser( ValueCallback<Uri> uploadMsg, String acceptType ) 
    {  
        context.mUploadMessage = uploadMsg;  
        Intent i = new Intent(Intent.ACTION_GET_CONTENT);  
        i.addCategory(Intent.CATEGORY_OPENABLE);  
        i.setType("image/*");  
        context.startActivityForResult( Intent.createChooser( i, "File Chooser" ), MainActivity.FILECHOOSER_RESULTCODE );  
    }

    // For Android < 3.0
    public void openFileChooser( ValueCallback<Uri> uploadMsg ) 
    {
        openFileChooser( uploadMsg, "" );
    }
}