How to attach a file to particular fieldname?

I am able to upload a file through rest api (Using postmon) but cant able to attach a file to particular field.
Here in image, i want to attach a file to pan card field and the field name is pan_card_image.
Passing this variable through postmon :-
doctype ,docname,is_private,filename,filedata(base64) ,file_size,from_form,cmd: uploadfile
Using this field i can upload the file but cant not attach to particular field like pancard or adharcard.

IMO you should try using a child table with a select field (PAN, GSTIN etc) and a corresponding attach field for file upload.

@mukesh_shinde Have you resolved this?, even I am facing same issue.