@Ted *******
I had the same issue when applying from the UK. I got my bank to print statements proving I had the income in excess of 65,000 Baht a month. They stamped each page and signed each stamp.
I then made 2 copies of all my supporting documents (bank statements, police check, medical....) and had the copies noterised. I applied in person at the Thai Embassy in London. If you are applying on line I would assume it is these noterised copies what you need to upload.