Symfony 6: embedded form collection, add only new elements and omit existing elements - symfony

Question

I have a FormType which contains an embedded collection of another FormType, all is working properly and done according to How to Embed a Collection of Forms.
My embedded FormType is an attachment list, which has title and file upload.
AttachmentType:
builder
->add('title', null, ['label' => 'Title', 'required' => true])
->add('attachmentFile', FileType::class, [
'label' => 'File',
'mapped' => false,
'required' => true,
'constraints' => [
new File([
'maxSize' => '1024k',
])
],
])
The problem is, when editing the parent object, I want to allow new attachments but I don't want to edit previous attachments, as it would be forcing me to upload the same files every time I edit the parent object.
All elements and javascript are done according to the docs linked, I don't find any property for CollectionType that would allow add but wouldn't allow edit or something similar. The javascript seems too dependent on external stuff to allow some modification.
Javascript
// assets/controllers/form-collection_controller.js
import { Controller } from '#hotwired/stimulus';
export default class extends Controller {
static targets = ["collectionContainer"]
static values = {
index : Number,
prototype: String,
}
addCollectionElement(event)
{
const item = document.createElement('li');
item.innerHTML = this.prototypeValue.replace(/__name__/g, this.indexValue);
this.collectionContainerTarget.appendChild(item);
this.indexValue++;
}
}
Twig template
<div {{ stimulus_controller('form-collection') }}
data-form-collection-index-value="{{ form.attachments|length > 0 ? form.attachments|last.vars.name + 1 : 0 }}"
data-form-collection-prototype-value="{{ form_widget(form.attachments.vars.prototype)|e('html_attr') }}"
>
<ul {{ stimulus_target('form-collection', 'collectionContainer') }}></ul>
<button type="button" class="btn btn-secondary" {{ stimulus_action('form-collection', 'addCollectionElement') }}>Añadir adjunto</button>
</div>