Apache2 returns result type html for png

Question

I try to install icinga2 and icingaweb2 on a Ubuntu 16.04 with Apache 2.4.18 With icinga2 everything works fine.

For icingaweb2 I used the official way with package repositories (02-Installation.md), that created the folder under /usr/share/icingaweb2 and the alias config under /etc/apache2/conf-available.

When I open the website the icingaweb2 page is visible, but the png pictures not. Inspecting the response shows return code 200 for the png but with 0 KB an type html and not png.

Maybe there is something wrong with the alias config:

Alias /icingaweb2 "/usr/share/icingaweb2/public"

<Directory "/usr/share/icingaweb2/public">
  Options SymLinksIfOwnerMatch
  AllowOverride None

  SetEnv ICINGAWEB_CONFIGDIR "/etc/icingaweb2"

  EnableSendfile Off
  AddHandler application/x-httpd-php .php

  <IfModule mod_rewrite.c>
    RewriteEngine on
    RewriteBase /icingaweb2/
    RewriteCond %{REQUEST_URI} !(\.css|\.js|\.png|\.jpg|\.gif|robots\.txt)$ [NC]
    RewriteCond %{REQUEST_FILENAME} -s [OR]
    RewriteCond %{REQUEST_FILENAME} -l [OR]
    RewriteCond %{REQUEST_FILENAME} -d
    RewriteRule ^.*$ - [NC,L]
    RewriteRule ^.*$ index.php [NC,L]
  </IfModule>

  <IfModule !mod_rewrite.c>
    DirectoryIndex error_norewrite.html
    ErrorDocument 404 /error_norewrite.html
  </IfModule>

  <IfModule mod_fcgid.c>
    SetHandler fcgid-script
    FCGIWrapper /var/www/cgi-bin/cgi_wrapper/cgi_wrapper .php
    Options +ExecCGI
   </IfModule>

</Directory>

Any ideas?