Flask-WTForms FileField Not Validating - flask

Question

I'm trying to build an app to upload a file to a web server using Flask and WTForms' FileField form field. The post is going through successfully but I am curious as to why form.validate_on_submit() fails each time even though the particular validators all succeed. Here is my code for the form (forms.py), the app (main.py) and the html template (upload.html).
### forms.py
from flask.ext.wtf import Form
from flask.ext.wtf.html5 import EmailField
from flask.ext.wtf.file import FileField, FileRequired, FileAllowed
from wtforms import validators, ValidationError, SubmitField
class UploadForm(Form):
presentation = FileField('Presentation in Image Format', validators=[FileRequired(), FileAllowed(['jpg', 'png'], 'Images only!')])
submit = SubmitField("Send")
### main.py
from forms import UploadForm
from flask import render_template, url_for, redirect, send_from_directory
#app.route('/upload/', methods=('GET', 'POST'))
def upload():
form = UploadForm()
if form.validate_on_submit():
filename = secure_filename(form.presentation.file.filename)
print filename
form.presentation.file.save(os.path.join('uploads/', filename))
return redirect(url_for('uploads', filename=filename))
filename = None
return render_template('upload.html', form=form, filename=filename)
#app.route('/uploads/<filename>')
def uploaded_file(filename):
return send_from_directory(app.config['UPLOAD_FOLDER'], filename)
### upload.html
{% for message in form.presentation.errors %}
<div class="flash">{{ message }}</div>
{% endfor %}
<form action="/upload/" method="POST" enctype="multipart/form-data">
{{ form.presentation.label }}
{{ form.presentation }}
{{ form.submit}}
</form>
Does anyone know why this might not be validating? Or should I not be using validate_on_submit()?

Answer 1

Flask-WTF enables CRSF by default, if you print the form.errors, you will get a message that tells you a csrf token is required.
The solution is simple, put {{ form.csrf_token }} in your template, or disable CSRF of the form, but you really shouldn't do such thing.
<form action="/upload/" method="POST" enctype="multipart/form-data">
{{ form.presentation.label }}
{{ form.presentation }}
{{ form.csrf_token }}
{{ form.submit}}
</form>
There is also a quick way to add every hidden fields to the form hidden_tags:
<form action="/upload/" method="POST" enctype="multipart/form-data">
{{ form.presentation.label }}
{{ form.presentation }}
{{ form.hidden_tag() }}
{{ form.submit}}
</form>