If a File Extension Is Not in the File Name?

If a file extension is not in the file name, and not because it is hidden, such as a .pdf, how do you determine its file type?

Why post a question like this Anonymously? Please be real. I’d simply downvote because it is a needlessly anonymous question. Needlessly because it is a technical, not personal question. But really, “no extension”? If it is because you cannot SEE the extension, when you can see the extensions of other files in Windows (pre-Windows 10)/File (Windows 10) Explorer? If NO extensions are showing, it is because you have that option turned on. Change it. If it actually has NO extension - Do.Not.Double-Click to open - unless you totally trust the source. It could be a malware executable. Open notepad and then drop the file on it. Notepad will not execute a program. If you can read text in the file, you have a good hint. For example, PDF files have %PDF as the first few characters*. (Though I just tried this on one with notepad and got Chinese characters! Opening it with a text editor showed the %PDF.) The Office *x files, such as docx, xlsx, etc, are actually zip archives, so opened in notepad, the first two characters are PK (a throwback to the PKZIP program.) Unzipping a zip archive will not execute a contained program, so you could chance unzipping, with File Explorer or an archiving program such as 7zip. If you see gobbledygook characters and nothing recognizable, it is probably binary, and hence could be an executable. Go back to the source. If it came by email, ask the sender. If you downloaded it, recheck the site or try to get it from another site, or the provider. *PDF files can contain embedded malware executables, so still be cautious if the source is suspicious. I’m answering “How do I open a file with no extension?” posted anonymously, with the hope that it may help and protect someone else, sometime.