Submitting file on Heroku / Flask Production causing Error, works locally

Question

I have a web form that allows people to upload their resume and enter their email, which once submitted, emails the info to me (resume as an email attachment). It works locally but causes as error in production - just says "internal server error". It's on Heroku / Flask and I'm using Flask Mail for the email.

Here is the code. My hunch is that it's the resume path is not being picked up on production but I'm not sure.

app.config["UPLOAD_FOLDER"] = "app/resumes/"

# --------------> Cold Resume Submission

@app.route('/', methods=["GET", "POST"])
def homepage():
    if request.method == 'GET':
        return render_template('home.html')
    else:
        cold_email = request.form["cold_email"]
        file = request.files['cold_resume']
        if file and allowed_file(file.filename) and cold_email:
            filename = secure_filename(file.filename)
            file_path = os.path.join(app.config['UPLOAD_FOLDER'], filename)
            print "file_path", file_path
            file.save(file_path)
            send_email(subject="Cold Resume...", 
                sender="LHV TalentTracker", 
                recipients=email_to_admin, 
                html_body="Cold Application from Homepage from {0}".format(cold_email),
                attachment_path=file_path)
            os.remove(file_path)    
        else:
            confirmation = "File or Email invalid"
            return render_template("confirmation.html", confirmation=confirmation)
    confirmation = "Thanks, We'll be in touch!"
    return render_template("confirmation.html", confirmation=confirmation)

------UPDATE----

I wrapped everything in a try / except block and received the following error...

homepage email error: <type 'exceptions.Exception'> [Errno 2] No such file or directory: 'app/resumes/Suraj_Kapoor_-_Cover_Letter.pdf' <type 'exceptions.IOError'> 

...which confirms my suspicion that the file path is not being picked up, but I'm unclear as to what's wrong with it. I do have an app/resumes folder present.