How macOS actually works
Finder hides extensions by default, which causes a specific and avoidable class of confusion.
Finder → Settings → Advanced → Show all filename extensions. Do this on a new Mac before anything else.
Without it, report.pdf and report.pdf.exe look identical, which is exactly the ambiguity malware relies on.
⌘I on a file → Hide extension checkbox in the Name & Extension section.
macOS records a Uniform Type Identifier alongside the name. Renaming photo.png to photo.jpg does not convert anything — it produces a PNG with a misleading name, and most apps will open it correctly anyway by reading the file's contents.
This is why Finder's Kind filter finds files whose extensions are wrong, and a name-based *.jpg search does not.
⌘I → Open with → choose an app → Change All…. This edits Launch Services, not the file.
Common on Unix — README, Makefile, .gitignore. macOS infers the type from contents where it can. Quick Look often refuses to preview them without the qlstephen plugin.
Extension search (*.pdf) and kind search (Kind is PDF) give different results, and both are correct for different questions. Use kind when hunting for content of a type; use extension when cleaning up by suffix.
No. It changes the name only. The contents and the file's real type are unchanged.
Extensions are optional on macOS. The system uses type metadata and content inspection instead.
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