当我单击“选择要上传的文件”（即输入类型=文件）时，从单击按钮和选择文件到在按钮旁边显示所选文件之间存在延迟。浏览器是否正在尝试将文件加载到浏览器中？为什么有延迟。
接下来，我如何显示“请稍候..”消息立即地选择文件后。我尝试了各种 JQ 选项，似乎都是在初始延迟后触发的（正如我所说，可能是浏览器正在尝试加载不确定的文件）我想用加载器小部件/消息来覆盖延迟。
请帮忙。
方法是使用button到要触发的元素click on input type="file"兄弟姐妹Open File对话;将“正在加载”小部件附加到文档；利用.one()附上focus事件至$(window)删除“正在加载”小部件window恢复focus用户选择文件或关闭后Open File dialog.
$("button").click(function() {
var spinner = $("<img />", {
"id": "spinner",
"src": "data:image/gif;charset=binary;base64,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"
});
$(this).after(spinner).nextAll("input:first").click();
$(window).one("focus", function() {
$("#spinner").detach()
})
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js">
</script>
<button>choose file</button>
<input type="file" style="opacity:0;"/>
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