[krutoy]

nerv_,
Это можно и проще сделать

Object.prototype.extend=function(src){
   for(var i in src){if(i in this) continue; this[i]=src[i]}
}

function FormField(data) {
  this._d = data;
}
    
function FormFile(data, opts) {
  FormField.call(this, data);
  this.opts = opts;
}
    
FormFile.prototype.extend(FormField.prototype)
    
o=new FormField()
console.log(o.constructor); // [Function: FormField]
 
o=new FormFile()
console.log(o.constructor); // [Function: FormFile]

Но, тем не менее...