fix the downcast for formdata try_as_web_event

[rogusdev]

Demo app to prove this change:

use dioxus::prelude::*;
use dioxus_logger::tracing::{info, Level};
use dioxus::web::WebEventExt;

fn main() {
    dioxus_logger::init(Level::INFO).expect("failed to init logger");
    info!("starting app");
    launch(App);
}

#[component]
fn App() -> Element {
    rsx! {
        input {
            type: "file",
            onchange: move |evt| {
                info!("onchange");
                if let Some(evt) = evt.try_as_web_event() {
                    info!("web event");
                }
            },
        }
    }
}

Before my change, selecting a file would only log "onchange". After my change you get both "onchange" and "web event" as you would expect.

I suspect this is a parallel problem to #3137 and possibly needs to be updated in more places as well.

[rogusdev]

I am suspicious that WebImageEvent needs to be updated in the same way (I am not qualified to test making such a change tho), however all the other impls seem to be returning the web_sys event type correctly, to my eyes.

[ealmloff]

Thanks!