[Python] Flask not throwing wrong file format error [closed]

I am creating a upload portal that allows users to upload a csv or text file. This is the code -

{% extends "layout.html" %}

{% block content %}
<div class="content-section">
<form method="POST" action="" enctype="multipart/form-data">
{{ form.hidden_tag() }}
<fieldset class="form-group">
<legend class = "border-bottom mb-4">Login</legend>
<div class="form-group">
{{ form.delete_command.label(class="form-control-label") }}
{{ form.delete_command(class="form-control form-control-lg") }}
{% if form.delete_command.errors %}
{% for error in form.delete_command.errors %}
<span>{{ error }}</span></br>
{% endfor %}
{% endif %}
</div>
</fieldset>
<div class="form-group">
{{ form.submit(class="btn btn-outline-info") }}
</div>
</form>
</div>
{% endblock content %}


My layout.html has this code block to display flash messages -

{% with messages=get_flashed_messages(with_categories=true) %}
{% if messages %}
{% for category,message in messages %}
<div class="alert alert-{{ category }}">
{{ message }}
</div>
{% endfor %}
{% endif %}
{% endwith %}
{% block content %}{% endblock %}


What am I missing?

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