205 is the file extension that is characteristic of a program called Winfunktion Mathematik v8.0. The program version is indicated here on purpose, as there is no information that other versions of this software also support this format. It is a mathematical software from the German company bhv Publishing with many advanced functions.

205 are files used to store data related to Julia fractals. These are characteristic structures that Winfunktion Mathematik can visualize with the given data.