e.target 属性类型取决于你所在的元素返回 getElementById（...）。 files 是输入的属性元素： https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement

The e.target property type depends on the element you are returning on getElementById(...). files is a property of input element: https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement

在这种情况下，TypeScript编译器不知道您正在返回输入元素，而我们没有事件特定于此的类。因此，您可以创建一个类似下面的代码：

In this case, the TypeScript compiler doesn't know you are returning an input element and we dont have an Event class specific for this. So, you can create one like the following code:

interface HTMLInputEvent extends Event {
    target: HTMLInputElement & EventTarget;
}

document.getElementById("customimage").onchange = function(e?: HTMLInputEvent) {
    let files: any = e.target.files[0]; 
    //...
}