Social photo post: Do not overwrite image alt, when not empty

When a user creates a post with a photo, no user interface is presented to enter an Alternative text for the image.
This is a great lean solution to make it as easy as possible for the user to publish pictures.

Instead the filename is used to fill the Alternative text. This happens without taking care, if an Alternative text exists or not. Usually there cant be any text, because no UI exists to fill it in. But what, if a service has already created a better Alternative text?

It would be great to have a simple check for it before overwriting.