custom-element.html

<template>
  <input type="file" accept="image/*" multiple files.bind="images"/>
</template>

custom-element.js

import {customElement, bindable} from 'aurelia-framework';

@customElement('custom-element')
export class CustomElement {
  
  @bindable images = [];
}

home.html

<template>
  <require from="custom-element"/>
  <custom-element images.bind="files"></custom-element>
  <button click.delegate="save()">Speichern</button>
</template>

home.js

export class Home {
  
  files = [];
  
  save() {
    // After selecting files this.files is still empty
    console.log(files);
  }
}