Can't unit-test image upload with python

I have a function that receives data from a form via POST. The function works fine bud I don't know how to test it.

I don't know how to send my function an image through the test.

Im guessing I have to create a temporary file but haven't been able to make it work. Maybe there is a better way of doing this?

This is the function I wanna test (app.py):

@app.route('/registering', methods=['GET', 'POST'])
def registering():
    if request.method == 'POST':
        userCheck = request.form['username']
        userCheck2 = request.form['email']
        userCheck3 = request.form['password']
        userCheck4 = request.form['passwordCheck']
        userCheck5 = request.form['phone']
        if userCheck == "":
            flash('"Username required"')
            return render_template('register.html')
        userName = users.query.filter_by(userName=userCheck).first()
        if userName:
            flash('"Username Taken'"")
            return render_template('register.html')
        if userCheck2 == "":
            flash('"Email required"')
            return render_template('register.html')
        if userCheck5 == "":
            flash('"Phone required"')
            return render_template('register.html')
        if userCheck3 == "":
            flash('"Password required"')
            return render_template('register.html')
        if userCheck4 == "":
            flash('"Confirm password"')
            return render_template('register.html')

        file = request.files['file']
        if file and allowed_file(file.filename):

            if userCheck3 == userCheck4:
                newUser = users(userCheck, userCheck2, userCheck5, userCheck3)
                db.session.add(newUser)
                db.session.commit()
                userName = users.query.filter_by(
                    userName=userCheck, userPass=userCheck3).first()

                session['logged_in'] = True
                session['user_id'] = userName.id

                #image part
                filename = str(userName.id)
                file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename + ".jpg"))

                flash('"Registered Successfully"')
                return redirect(url_for('friendList'))
            else:
                flash('"Retype passwords"')
                return render_template('register.html')
        else:
            flash('"Add an image (.jpg)"')
            return render_template('register.html')

    else:
        return render_template('register.html')

def allowed_file(filename):
    return '.' in filename and filename.rsplit('.', 1)[1] in ALLOWED_EXTENSIONS

Here is my test (app_tests.py):

import os
import app
import unittest
import tempfile

class AppTestCase(unittest.TestCase):

    def setUp(self):
        self.db_fd, app.app.config['DATABASE'] = tempfile.mkstemp()
        app.app.config['TESTING'] = True
        self.app = app.app.test_client()

    def tearDown(self):
        os.close(self.db_fd)
        os.unlink(app.app.config['DATABASE'])


    def registering(self, username, email, password, passwordCheck, phone, file):
    return self.app.post('/registering', data=dict(
        username=username, 
        email=email, 
        password=password, 
        passwordCheck=passwordCheck, 
        phone=phone,
        file=file
    ), follow_redirects=True)

    def test_registering(self):
        #successfully registered

        rv = self.registering('TestUser', 'test@test.com', 'passwordTest', 'passwordTest', '900102030')
        assert 'Registered Successfully' in rv.data
        #existing username/user
        rv = self.registering('Hulda', 'hulda@hulda.com', 'passwordHulda', 'passwordHulda', '900102030')
        assert 'Username Taken' in rv.data
         #no username
        rv = self.registering('', 'santana@santana.com', 'passwordSantana', 'passwordSantana', '900102030')
        assert 'Username required' in rv.data
        #no email
        rv = self.registering('Santana', '', 'passwordSantana', 'passwordSantana', '900102030')
        assert 'Email required' in rv.data
        #no password
        rv = self.registering('Santana', 'santana@santana.com', '', 'passwordSantana', '900102030')
        assert 'Password required' in rv.data
        #no password confirmation
        rv = self.registering('Santana', 'santana@santana.com', 'passwordSantana', '', '900102030')
        assert 'Confirm password' in rv.data
        #no password match
        rv = self.registering('Santana', 'santana@santana.com', 'passwordSantana', 'passwordSsssssntana', '900102030')
        assert 'Retype passwords' in rv.data
        #no phone
        rv = self.registering('Santana', 'santana@santana.com', 'passwordSantana', 'passwordSantana', '')
        assert 'Phone required' in rv.data