In `media.model.PostMedia`, for these 2 scenarios:

* `src` is set, and 'Add a Source' results in the same file (or a file with the same extension) being added
* `src` is set, and 'Replace Audio|Video' is used, which will add a model property named by the attachment's extension

... call `model.unset( 'src' )`.

See #27389.



git-svn-id: https://develop.svn.wordpress.org/trunk@27536 602fd350-edb4-49c9-b593-d223f7449a82