A "???" file is generally not a real extension because the extension is missing or the file is partial, so turning on "File name extensions" shows whether it’s actually .pdf, .zip, .mp4, and if none appears, it may be intentionally extensionless; checking file size distinguishes broken downloads from real data, and magic-byte checks via Notepad—looking for "%PDF-", "PK", "MZ"—can reveal its type, with the containing folder providing additional hints, and trying common apps like a PDF reader, 7-Zip, or VLC often confirms what it is before you rename it properly.
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web site. When I said "???" isn’t a true file type, I meant it’s simply what your system displays when it doesn’t know how to classify a file because the extension is wrong, since the OS depends on that extension to assign icons and default apps; if a file has no extension, uses a rare one, was renamed incorrectly, or is partially downloaded or corrupted, Windows may show "???" even though the file actually has a real internal format, which you can figure out by revealing the extension, checking size, looking at its first bytes (like %PDF- or PK), and noting where it came from before opening it properly.
When I say "???" is a label and not a true extension, I mean the system is simply displaying it as a generic "unknown" type instead of reading it as part of the filename, because the extension—like .pdf or .jpg—is what actually tells the OS how to classify the file, while labels such as "PDF Document," "JPEG Image," or "???" are only on-screen descriptions, so if the OS can’t interpret the file due to a mismatched extension or corruption, it may show "???" even though the real format can still be identified through the filename, size, or magic bytes.
When I say "???" shows up due to the system not knowing the type, I mean the OS needs a dependable clue—typically the characters after the last dot—to classify the file, and when that extension is missing, or when the file’s contents contradict it, or when corruption prevents header reading, the OS defaults to "???," with certain apps doing the same when no metadata or association exists, though the underlying format can still be found via the extension, file size, or first bytes like %PDF-, PK, or MZ.
Think of it like this: the file extension serves as the computer’s hint for selecting the right app—PDF reader, image viewer, archive tool—and "???" just means that label is obscured, leaving the OS uncertain even though the file itself may be fine, and checking the extension, file size, and internal magic bytes usually clarifies what the file truly is.