How to create/upload representation through API?

If the file exists serve side or is accessible by the server you can use Java api to create new application data then add it to a content holder, in this case a representation.

To upload from client side would need some additional cusotmisation of the UI to add a button to open the file browser etc etc

Hello @TT_9345405 

I would start to study how a AfterLoader works.

You can check the source code and learn how to add a additional files to a representation.

During a changing name the file is stored again to a representation.

From that point you can create new representation by general API to store a new object, and then attach a file as primary representation content (ContentRoleType.PRODUCT_VIEW_ED). 

https://www.ptc.com/en/support/article/cs252900

PetrH