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A friend of mine who uses a Mac gave me a whole raft of .jpg files on a thumb drive (which appears to be formatted as FAT32). I can't open them on any app on my Windows 10 PC, including Windows Photos or Photoshop. I get "it appears we don't support this file format" in Windows Photos

When I look at them in a hex editor they are very different from the normal JPEGs on my PC from Photoshop or my cameras. My normal JPEGs all start with FF D8 FF ... and then Exif or JFIF data starts around 06. But in the unreadable ones from the Mac there is no FF D8 FF and where I expect to see the Exif data it says "Mac OS X". Are JPEG files on a mac "different"? or what am I seeing here?

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    I suspect they have a codec on them that is not on your system such as HEIF, although normally they have the extension .heic
    – LPChip
    Sep 22, 2022 at 19:38
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    … though HEIC photos should actually have .heic as the extension. They do straight from an iPhone. Inherently, there's no difference in a 'jpg' from a Mac or anything else. I'd suspect if not .heic, then perhaps just a user error. Ask them what they exported from & how. Recent versions of Ps support heic on Mac, idk about on Win.
    – Tetsujin
    Sep 22, 2022 at 19:43
  • In Linux file is the tool. I don't know if Windows has anything like it. If you can boot Linux without too much hassle, do it and see what file tells you.
    – Kamil Maciorowski
    Sep 22, 2022 at 19:44

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It turns out that the answer is that these are not really JPEG files despite ending in .jpg.

They are something called "OSX Resource Forks". A Mac user informs me that they are generated automatically by the Mac OS along with the JPEG file. They can be recognized by a "._" at the beginning of the filename. Furthermore the person who gave me these files can't even see them on his Mac because they are hidden, so Mac users don't even know when they're doing this.

Source: https://community.adobe.com/t5/photoshop-ecosystem-discussions/can-t-open-jpg-files-made-on-a-mac/m-p/13218271/thread-id/673531#M673558

I have no information about what function those files serve or why they end in an industry-standard file-extension (.jpg) despite not actually being JPEG files, or how a non-geek user is supposed to know this.

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    – Community Bot
    Sep 22, 2022 at 22:33
  • OK I added my source.
    – user316117
    Sep 22, 2022 at 23:32
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    Your supplier can't see them on the Mac because they don't exist on the Mac, they're generated when copying to a disk formatted with a file system that cannot support permissions, ACLs or extended attributes. It's done so the files can be re-assembled correctly on another Mac, without losing anything. They're similar to windows alternate streams, except windows just throws them away if you move them to any FAT formatted disk. Files beginning with a dot should ordinarily be invisible on any OS, by convention.
    – Tetsujin
    Sep 24, 2022 at 11:04

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