Dear Kali, you needn't upload an original caste certificate. On the official website, to the right bottom of the website, there is a column called Certificate. Under that, there are relevant Category certificates in downloadable pdf format.  You need to download that pdf file relevant to  your Category, fill that up and get it self attested and then upload it, The instructions are clear about it. Please try this procedure, If you are not able to upload this way, please raise a Candidate query and check with the helpdesk.