Since you are applying for a government job. fill up the essential entries on line as per requirement and then in support of your passing out Diploma, you have to upload the scanned copy of the certificate and in absence of that one, you may upload the Xerox copy of the certificate indicating your validity for the said post.
Once you upload Xerox copy of your passed out certificate coupled with the filling up the form, you have to submit the same on line and at the end of the process, you will be provided with an acknowledgment slip which should be retained for the official purpose.
However, at the time of document - verification, you must produce all the original certificates failing which it might be a costly affair. Your original certificate would include your passing out class ten board from a recognised institution followed by your class twelve certificate of the recognised board, Final Diploma Certificates, mark - sheets of class ten and twelve certificates and the charecter certificate issued by the your Polytechnic etc.
Hence ensure that all such documents are available to you at the time of document - verification.