Error when uploading images containing "conf" as first 4 letters

[chuckymendoza]

I have discovered a strange problem with two completely different PW websites (PW 3.0.123) that are hosted on different hosting services. 

When I upload images whose first 4 initial letters result in the word "conf", the images are uploaded but not displayed in the backend.  They are also displayed as a "broken link" in the frontend. I cannot edit them in the backend either. Example files could be "conference.jpg" or "confirmation.jpg" or just "conf.jpg".
I could not detect this problem on my virtual server (MAMP).

I use a simple image-field with the default setting.

If I rename the same image (as shown in the screenshot here) as a test, a preview of the image is generated without any problems.

Does anybody have an idea what this could be due to?

Thanks so much,

Thomas

[Macrura]

possibly mod security