Modified fileupload_find and updated README

**I discovered strange bug (?) in fileupload_find function:**
It returns only the first record with the given guid.
I was implementing validation and rails-uploader was showing only the first uploaded image in invalid form (after page reload).
I removed *.first*, and now it works as it should.
But I don't know if it's a bug, or it was specifically written like this.

--

**Also, I noticed that _download.html.erb template and returned JSON array with files doesn't match:**
Function *render_json* in attachments_controller has this line:
```hash_or_object.to_json(:root => false)```
So, *create* function, that uses this function, returns array of serialized objects and it doesn't have the structure that is used in _download.html.erb partial. (for example, there's no filename field).
In theory, you can create needed fields as instance methods, but I think that *serializable_hash* is a better solution.