So, in my usual rather perverse fashion, I am force-fitting Prototype into Bootstrap, since I have a gigantic mental block where jQuery should be. It actually works pretty well. Here are some widgets I have made over the course of building projects with these two frameworks.

file_decorator.js

document.observe('dom:loaded', function(){
  var file_decorator = function(){
    $$('input.file').each(function(elm){
      var elm = elm.hide();
      if(elm.decorated) return;
      elm['decorated'] = true;
      var group = new Element('div', {
        'class': 'input-append'
      });
      var txt = new Element('input', {
        type: 'text',
        'class': 'input'
      });
      var btn = new Element('a', {
        'class': 'btn'
      }).update('Browse').observe('click', function(evt){
        elm.click();
      });
      elm.insert({after: group});
      group.insert(txt);
      group.insert(btn);
      elm.observe('change', function(evt){
        txt.setValue(this.getValue().split(/[\\\/]/).last());
      });
    });
  };
  file_decorator();
  document.observe('DOMSubtreeModified', file_decorator);
});