Flask how to process file and data in form

Question

I'm trying to read a form containing an uploaded file in form with Flask 0.11.1 in Python 2.7.6.

app.route("/registration_submission")
def registration_submission():  
    print request.files
    print request.form
    print request.data
    # sth else...

Several methods I have tried give following results:
(1) Pure HTML form. It is like the following. The name field is contained according to the W3S Note, and the enctype is set to multipart/form-data:

<form id="register-form" action="/registration_submission" method="post" enctype="multipart/form-data">
    <div><input type="text"id="firstname" name="firstname"></div>
    <div><input type="text"id="lastname" name="lastname"></div>
    <div><input type="file" id="resume" name="resume"></div>
    <button type="submit" id="send">Submit</button>
</form>

• The file can be retrieved from request.form['filename']
• request.data is empty. It is usually empty according to this and this.

(2) jQuery AJAX POST. In addition to the HTML form used above, I set AJAX processData to false.

    $.ajax({
    type: "POST",
    url: "/registration_submission",
    data: submission_data,
    processData: false,
    error: function(response, error) {
            ;
        },
    success: function(data, code, jqxhr) {
            ;
        }
    });

• If the submission_data was acquired by var submission_data = $('#form').serialize() then:
  • request.files is empty
  • request.form contains all other information except the uploaded file, in pretty format
  • request.data is empty. But if I call it after calling request.get_data() then request.data is a nicely formatted url-style string
• If, however, I use var submission_data = new FormData($('#form')[0]) then:
  • request.files is empty
  • request.form contains a messy blurb of encoded characters
  • request.form.to_dict() returns a dictionary of a messy blurb
  • request.data is empty. If I look for request.data after calling request.get_data() then it seems like request.data contains several thousand lines of information containing almost the whole html page.

Any idea to make jQuery AJAX form containing file and data retrievable in Flask? (I can't use option (1) since I need to do some checking before submitting the form). Thanks!