How do you re-initialize an using javascript/jQuery?

Browsers set the defaultValue property of <input> DOM nodes to the original content of the value attribute from the HTML, so you can do something like

in an event handler or whatever.

Your question title involves a "file" <input> element, for which value attributes are meaningless. There is no "default value" for a "file" <input>, so setting the value to the empty string is the thing to do.