In order to increase the chances of genuine and official documentation (eg bank statements, mortgage offers, solicitors letters etc) being uploaded, and to reduce the possibility of malware being passed into the system, we only accept pdfs and photos of documents (.jpg & .png).
Most notably this means we do not accept .csv, .doc or .xls files - this is because (i) these file types are easily manipulated prior to upload, and (ii) malware is easily inserted into these documents.
If the document you are trying to upload is not in one of those formats, you should convert it to pdf (File --> Save as...) or take a photo of it before uploading.