How can I validate file uploads in Flask-WTF?

How can I validate file uploads in Flask-WTF?

You can use Flask-WTF's FileField along with validators like FileRequired and FileAllowed to validate file uploads.

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

Sure! Here's a simple example:

from flask_wtf import FlaskForm
from wtforms import FileField, SubmitField
from wtforms.validators import FileRequired, FileAllowed

class UploadForm(FlaskForm):
    file = FileField('Upload File', validators=[FileRequired(), FileAllowed(['jpg', 'png'], 'Images only!')])
    submit = SubmitField('Upload')

What happens if the file type is not allowed?

If the file type is not allowed, Flask-WTF will raise a validation error, and you can display a message to the user.

How can I customize the error message?

You can customize the error message by passing a message parameter to the validator, like this:

FileAllowed(['jpg', 'png'], 'Only JPEG and PNG files are allowed!')