When I make the comment "Missing file extension" on an assignment, it means that the portion of the file name, which is usually three or four characters and comes after the period, is missing.

Why does this matter? Computers use the extension to help determine what program is used to open the file. For example, doc and docx belong to Microsoft Word, psd is used by Adobe's Photoshop. Without these extensions, it can be impossible to know what program will open the file. When the extension is missing, I take my best guess as to what the file is, rename the file, and attempt to open it with the appropriate program. If I'm wrong, the file can be corrupted.

This brings up another point: it's best not to add any periods to a file name as this may confuse the save process and eliminate the extension.

Keep it simple, don't change any part of a file name to the right of, and including the period. Don't add any periods to the file name.