How can I restrict file types in Flask-WTF file uploads?

How can I restrict file types in Flask-WTF file uploads?

You can restrict file types in Flask-WTF by using the FileField and adding a custom validator to check the file extension.

Can you show me an example of how to implement that?

Sure! You can create a custom validator like this:

from flask_wtf import FlaskForm
from wtforms import FileField, SubmitField
from wtforms.validators import ValidationError

class UploadForm(FlaskForm):
    file = FileField('Upload File')
    submit = SubmitField('Submit')

    def validate_file(form, field):
        allowed_extensions = {'pdf', 'png', 'jpg', 'jpeg'}
        if '.' not in field.data.filename:
            raise ValidationError('No file extension.')
        extension = field.data.filename.rsplit('.', 1)[1].lower()
        if extension not in allowed_extensions:
            raise ValidationError('File type not allowed.')

What happens if a user tries to upload a disallowed file type?

If a user tries to upload a disallowed file type, the ValidationError will be raised, and you can display an error message to the user.