Хорошо, я понял. Вот код для людей, желающих его.
class FileUploadForm(FlaskForm):
file = FileField('Choose File', validators=[FileRequired()])
submit = SubmitField('Upload')
@blueprint.route('/upload/', methods=['GET', 'POST'])
def upload():
form = FileUploadForm()
if form.validate_on_submit():
secure_name = secure_filename(form.file.data.filename)
new_file = File(secure_name, form.file.data.stream.read(),
mime_type=form.file.data.mimetype,
length=form.file.data.stream.tell())
db.session.add(new_file)
db.session.commit()
flash('File uploaded successfully !', 'success')
return redirect(url_for('manager.files.view', file_id=new_file.id))
return render_template('files/upload.html', form=form, title='Upload Files')