Pasting files from clipboard into browser

I'm trying to implement file pasting in the browser using the following code

document.onpaste = function (event) {
    for (var item of event.clipboardData.items) {
        if (item.kind == 'file')
            process(item.getAsFile());
    }
};

But I didn't get the file I pasted. Logging the DataTransferItem object item I noticed that its kind is not file but always string and it's a list of the copied files' names. Is there a problem in my code or this issue is related to my platform ? I am working with Firefox 104.0, Ubuntu 22.04 and Wayland