Image in Writer-document, right-click, Save Graphics.
When trying to save the image with a name containing one or more dots, anything
after the last dot is considered a spurious extension and dropped, and replaced
with the extension corresponding to the type selected from the drop-down list.

I encountered it when saving the images from a document using the
chapter/paragraph-numbering : "image 2.1.png", after a couple of saves I got the
message that the file already existed, and noticed that the intended name had
been corrupted to "image 2.png".
So I need to provide at least one other dot to preserve the desired name :
"image 2.1." or "image.2.1.png" or "image 2.1.rnd", al result in "image 2.1.png".