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.


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