This endpoint accepts file uploads for the specified event registration. You can PUT to this endpoint once for
each file upload field on your registration form by passing different fieldLabel values. Subsequent PUTs for
the same fieldLabel will overwrite the previously uploaded file. If your client does not automatically do so,
ensure you've included the file name in the Content-Disposition header (example:
Content-Disposition: form-data; name="fieldLabel"; filename="profile_pic.png").