How to restrict which files are selectable from WKWebView UI delegate in runOpenPanelWith? - wkwebview

Question

In a webpage I host in a WKWebView I want to allow selecting certain file types to open. For that I wrote a TS function to select files:
/**
* Allows the user to select a local file.
*
* #param contentType Restricts the file selection to specific file types.
* #param multiple If true, allows to select more than a single file.
*
* #returns A promise that resolves to a single file or a list of files.
*/
export const selectFile = (contentType: string, multiple: boolean): Promise<File | File[] | null> =>
new Promise((resolve): void => {
const input = document.createElement("input");
input.type = "file";
input.multiple = multiple;
input.accept = contentType;
document.body.appendChild(input);
input.onchange = (): void => {
const files = input.files ? Array.from(input.files) : null;
if (multiple) {
resolve(files);
} else {
resolve(files ? files[0] : null);
}
};
input.click();
document.body.removeChild(input);
});
As you can see there's a content type that can be specfied. In my WKWebView UI delegate I open a file selection panel:
/**
* Called when the web app requests a local file name.
*/
func webView(_ webView: WKWebView,
runOpenPanelWith parameters: WKOpenPanelParameters,
initiatedByFrame frame: WKFrameInfo,
completionHandler: #escaping ([URL]?) -> Void) {
let panel: NSOpenPanel = NSOpenPanel.init();
panel.allowsMultipleSelection = parameters.allowsMultipleSelection;
panel.canChooseDirectories = parameters.allowsDirectories;
panel.canChooseFiles = true;
panel.resolvesAliases = true;
panel.begin { (result) in
if result == .OK {
completionHandler(panel.urls)
} else {
completionHandler(nil)
}
}
}
However I cannot restrict the selectable files (according to the specified content type, e.g. application/json). The given parameters do not contain any value that would help here. So, how can I pass on the content type to the UI delegate function or otherwise make it aware of the possible file types?