untested pseudocode : use the dom to add <li> tags containing in <input type="file"> to an existing <ul id="fileList">tag
function addFileSelector ()
{
var ul= document.getElementById('fileList')
var newLI = document.createElement('LI')
var newText = document.createTextNode('File : ')
var newFileElement = document.createElement('INPUT')
newFileElement.id = 'blah'
newFileElement.setAttribute('type','file')
newFileElement.setAttribute('name','file[]')
newLI.appendChild(newText)
newLI.appendChild(newFileElement)
ul.appendChild(newLI)
}