The standard LaTeX \includegraphics{} command interprets all characters after the first dot in its file name argument as the file name’s “extension”, which sometimes leads to an “unknown graphics extension” error. This is inherited from the Windows world, and stems even from the DOS era, where programmers had the brilliant idea to encode file metadata, in this case the type of the file’s content, as part of the file’s name. One might wonder why they didn’t choose to encode, for example, the creation date of the file, or the name of the program used to create the file, as an “extension” to the file’s name, too.

I guess the creator of the graphics package was using a Windows (or DOS) PC, otherwise he would have chosen a more flexible way to determine the type of the graphics file. Luckily, there is a workaround: Just use

\usepackage{grffile}

in your LaTeX document.