Problems with gif files in apache web server

We have an Apache Webserver, Version 2.4.12 on a Windows 2008 machine.
We have gif images in different sub folders of htdocs, some in "icons" and some other in "ni5/customer/images". When I start our web application, the Google Chrome developer tools show something strange:

The one gif image from "ni5/customer/images" is loaded and the type is "gif", the other gif image from "icons" is NOT loaded and the type is "text/html".

When I open the second one with a image viewer, it is recognized as "gif".