Custom add_form template création issues

[Romia25]

Hello!
I'm new in development and am facing some issue about customisation.
I'm trying to upload file and I want to perform a lot of customisation like remove uuid automatically generated for filename so that the file upload process save the original filename in the db and the upload_folder. I somehow figure out how to do this by overriding pre_add and post_add methods. But the ultimate goal of this is to be able to show a modal dialog for user to confirm if he or she wants to continue upload process of an already existing file (from db) or cancel the whole thing. I know it's possible to have the dialog box because we see it when we want for example to delete a record. It's something similar I want to achieve. After clicking save button for the file to be uploaded, I want to have a similar dialog box in case the Same filename already exist in db.

So far I have override the pre_add method to get the original filename using the function in file manager that helps perform this. In this pre_add method I am also able to fetch the db table and know if a similar file was already upload. But then I'm confused.

From the post add side I used shutil to rename the file in the upload folder.
My big point of confusion is where and how could i make a modal dialog visible when the form is post and the filename already exist in the db.

So I decided to create my own upload form based on upload file model but here again fab can create the form by itself and I can't see where and how could I add maybe some JavaScript to have my modal dialog box showed up.
Going further in docs I saw I could define my own add_template and used extra_args for other elements that must be provided to the template. But here I don't know how to make my way.
Here is what I have done
add_temolate = 'form.html'
extra_args = {'form' : 'add_form'}

add_form was define early as MyUPloadFom

In the form.html
I have something like

{% extends 'appbuilder/general/model/add.html' %}

{% block body %}

{{ form.hidden_tag() }}

{{ form.name.label }}
{{ form.name() }}

{{ form.description.label }}
{{ form.description(s) }}

{{ form.submit() }}

{% endblock %}

But when I run the app I get hidden_tag() missing required positional argument self.
If I go up and comment this part and run the app again I get jinja2 error unboundedfield has no attribute label.

I really don't know what to do. I need guidance on how to achieve my goals.