If it were me, I would just treat this like a normal application, identify the resource-ids or texts of the fields you need to manipulate and do so like it was your application – click in the field, identify the resulting screen, click the appropriate image.
It may be that you can use Appium’s chrome driver for this but I haven’t used that yet.
Thanks Aleksei for your reply, but did not understnd how i should used it in my code, i too have the same scenario wherein i need to attach a photo from photo gallery. How do i get the coordinates of the photo gallery?