[Fixed]-Overridding Django Admin's object-tools bar for one Model

[A]

Move "django.contrib.admin" in your INSTALLED_APPS to end of INSTALLED_APPS.

You can make template file

<your_app>/templates/admin/<your_app>/<your_model>/change_list.html

source code:

{% extends "admin/change_list.html" %}

{% block object-tools-items %}
  <li>
    <a href="<your-action-url>" class="addlink">
     Upload file
    </a>
  </li>
  {{ block.super }}
{% endblock %}

[B]

Add change_list_template into your ModelAdmin.

class MyModelAdmin(admin.ModelAdmin):

    change_list_template = '<path-to-my-template>.html'

And write template like [A] source code.

It has become lot easier to accomplish what you’re looking for since this question has been asked the first time.

The Django documentation has a section on how to override admin templates. To add a button to the changelist object tools, follow these steps:

1. Copy Django’s version of the change_list_object_tools.html template file into your project’s or app’s template folder: templates/admin/<app>/<model>/change_list_object_tools.html.

   You can obtain the file form your virtual env’s site-packages folder:

   cp $VIRTUAL_ENV/lib/python3.8/site-packages/django/contrib/admin/templates/admin/change_list_tools.html templates/admin/$APP/$MODEL/ 
   

   Note that you might need to adjust the path to site-packages for your Python version.

2. Now open the template file. It will look liek this:

   {% load i18n admin_urls %}
   
   {% block object-tools-items %}
     {% if has_add_permission %}
     <li>
       {% url cl.opts|admin_urlname:'add' as add_url %}
       <a href="{% add_preserved_filters add_url is_popup to_field %}" class="addlink">
         {% blocktrans with cl.opts.verbose_name as name %}Add {{ name }}{% endblocktrans %}
       </a>
     </li>
     {% endif %}
   {% endblock %}
   

3. Add the link to your custom view:

     …
     {% if has_add_permission %}
     <li><a class="addlink" href="{% url 'upload-file' %}">Upload file</a></li>
     <li>
     …
   

That’s it! If you didn’t know: you can add custom views and URLs to a ModelAdmin using ModelAdmin.get_urls() (docs). To not have to hardcode your custom admin URLs, you can of course reverse them (docs).

You can try to use https://github.com/texastribune/django-object-actions.

This will allow you to add buttons with custom logic.
Though, I’m not sure whether you’ll be able to a buttom for your list screen, as I did it only for a model-edit page.