"file --extension" not work with PNG files

[Tóth Imre]

$ file l3e9wvdjsvQFD_7R\?24881501
l3e9wvdjsvQFD_7R?24881501: PNG image data, 300 x 169, 8-bit/color RGB, non-interlaced

$ file --extension l3e9wvdjsvQFD_7R\?24881501
l3e9wvdjsvQFD_7R?24881501:

$ file Q9z_VZVdrhVhQXKb\?24881501
Q9z_VZVdrhVhQXKb?24881501: JPEG image data, JFIF standard 1.01, resolution (DPI), density 96x96, segment length 16, comment: "CREATOR: gd-jpeg v1.0 (using IJG JPEG v80), default quality", baseline, precision 8, 1920x1080, frames 3

$ file --extension Q9z_VZVdrhVhQXKb\?24881501
Q9z_VZVdrhVhQXKb?24881501: jpeg/jpg/jpe/jfif

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: file 1:5.25-2ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-97.120-generic 4.4.87
Uname: Linux 4.4.0-97-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Oct 22 19:32:35 2017
Dependencies:
 gcc-6-base 6.0.1-0ubuntu1
 libc6 2.23-0ubuntu9
 libgcc1 1:6.0.1-0ubuntu1
 libmagic1 1:5.25-2ubuntu1
 zlib1g 1:1.2.8.dfsg-2ubuntu4.1
InstallationDate: Installed on 2015-02-02 (993 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
SourcePackage: file
UpgradeStatus: Upgraded to xenial on 2016-06-02 (506 days ago)