我希望它能满足您的要求：fiddle exapmle

<input type="file" id="mysignature_upload"/>

    $("#mysignature_upload").click(function (event) {
         $("#mysignature_upload").val("");
    });

    $("#mysignature_upload").on('change',function (event) {
        alert("selected file: " + $("#mysignature_upload").val());
    });

<script>
    $(document).ready(function({
      $("#mysignature_upload").click(function(event){
         event.preventDefault();
         $(this).val("");
      });
    });
</script>

顾名思义，onchange只有在字段值改变时才会被触发，如果每次用户选择文件时都需要做一些事情，那么您可以在click事件中重置字段值。

$("#mysignature_upload").click(function() {
  // clear the value
  $(this).val("");
 }
}

现在onchange每次都会被触发。

当选择大文件时，在文件对话框中单击“打开”和（类型文件）输入值更新之间有一个明显的延迟。虽然我使用以下两个事件来显示和隐藏“请等待”对话框，但您也可以在这些事件中“做其他事情”：

// Fires upon click of the file input ('Choose File' button)
    $('#selectedFile').on('click', function (event) {
        // Shows before file selection dialog and remains behind
        // the file selection dialog (which has a higher z-order)
        ShowPleaseWait();
    });

    // Fires the moment that the file input value itself is updated.
    // NOTE: This is NOT when the Open button is clicked ... this
    // is when the value of the file input actually updates (there is
    // a delay after clicking Open for a large file until the value
    // of the input actually, finally, updates...)
    $('#selectedFile').on('change', function (event) {
        HidePleaseWait();
    });