Html can’t load local video on iOS

ionic3, webview plugin 4.2.1

Html
```html

```

use fileTransfer plugin download video, get a url like:
```js
file:///var/.../Documents/video.mp4
// also tried toInternalURL()
```
error: Not allowed to load local resource

try
```js
this.videoUrl = this.win.Ionic.WebView.convertFileSrc(url); // ionic://localhost/_app_file_/var/.../Documents/video.mp4
// or use sanitizer
this.videoUrl = this.sanitizer.bypassSecurityTrustResourceUrl(this.win.Ionic.WebView.convertFileSrc(url));
```
both error: Failed to load resource: unsupported URL