extractContents: specify filename in extraction event

This ensures that, instead of specifying just the UUID of the source
package, its full name will be displayed as well.

e.g., before:

Unpacked from: {9d267635-fb89-445d-96a0-f7f814e714c5}

After:

Unpacked from: %transferDirectory%objects/package.zip (6a252aef-f737-4bcf-867f-c457c149426b)

Fixes #6576