fileinfo: Detection of overlay content type

Overlay data appended to executable files are not part of the executable image but they are often used to store configuration/another MZ(PE) files/payloads/etc. We should detect the content type of overlay in the same way as in #414. This information can be valuable for analysts and other tools where `fileinfo` is part of the toolchain.

Start by reading issue #414 and locating the existing fileinfo content-type detection it describes. Trace how executable overlay data is identified and reported, then verify that appended overlay content receives a type without changing the executable image result; the issue provides no file or test path.

Written by the indexing model from the issue text.