This is frequently resultant when use of double extensions (e.g. ".php.gif") bypasses a sanity check.
Resultant
(where the weakness is a quality issue that might indirectly make it easier to introduce security-relevant weaknesses or make them more difficult to detect)
This can be resultant from client-side enforcement (CWE-602); some products will include web script in web clients to check the filename, without verifying on the server side.