You may have saved a file with the wrong extension, for example a JPEG file with a .psd extension. You cannot open the file in Photoshop due to an error
- Could not complete your request because an unknown or invalid JPEG marker type is found
- Could not complete your request because it is not a valid Photoshop document
If you cannot remember the actual file type the file should be you can use the following method to determine the true file type of a document
Selecting File > Get Info... from the macOS Finder will not accurately tell you the true file type of a document as it only looks a the file name to make the determination. The method outlined below using the Terminal reads into the file header to get the true file type.
In this case, the file is named "mydocument.psd" but the file is actually a JPEG. Rename the file to "mydocument.jpg" to open in Photoshop.