Creating new file extension which shows an Image and selectable rectangles inside it

Hello folks,

I am developing a plugin which needs to create a custom file which shows an image ( mobile screen) and selectable rectangles( like icon selection) inside it. Which is the best way to implement such a file type ?

My file content is : Image binary and a JSON contains the locations of the rectangles.

Please suggest! Please advise on the the best way to do it.

Reagrds

Manoj