It is the '.doc' part. It tells the person that is getting the file what sort of file it is, and what program they can use to open it. e.g. .doc .pdf .ppt or .jpg

I'll rewrite this for you: "please include a file extension (e.g. essay.doc) ". as "please include a file with an extension of (.doc)." It's technically impossible to "include" file extensions because file extensions are only a part of a filename. You can upload complete files, but not just filenames or extensions. As I see it, the writer is asking you to include a MS Word '97 standard .doc file only. Not any other kind of file.

More an IT question than an English one, but in this example, it's the ".doc" part of the file name.