If you are updating your profile information as well as documents, make sure to enter all the required personal information to the profile form, so that the button "Next" that leads to document upload is enabled (until then it will stay greyed out).
On the document upload page make sure to upload both the POR and the ID documents before saving them by clicking on "Upload and Finish".
The maximum file size is 5MB and supported formats are .jpeg, .jpg, .pdf, .png. When the document is successfully uploaded it will be shown as:
Please do not forget to enter the ID document serial number or the button "Upload and Finish" will stay greyed out.