utiliza esto, pero tenes que ponerle un name al input file y todo entre etiquetas form
Código HTML:
<input type="button" value="Copiar" onclick="clipboardData.setData('Text',this.form.nombre_campo_file.value)"> Aca te paso que podes hacer con el objeto clipboardData
clearData([dataFormat]) Removes all data from the clipboard unless the string dataFormat is specified as "Text", "URL", "File", "HTML", or "Image", in which case only data of that kind is cleared. (IE5+ Windows)
getData(dataFormat) Gets data of the specified format from the clipboard and returns it as a string (of text, HTML, or a URL). (IE5+ Windows)
setData(dataFormat, data) Attempts to place the data given in string data (either text, HTML, or a URL) into the clipboard according to the data type specified in the string dataType (either Text, URL, File, HTML, or Image). Returns a Boolean indicating whether it was successful. (IE5+ Windows)