File Extensions

Requirements for Valid Extensions

    Valid file extensions are defined as those meeting the following criteria:
    1 Length
    1-12 characters in length.
    2 Content
    All characters must be in the range 'a' to 'z', 'A' to 'Z', '0' to '9', or one of '$', '%', '_', and '~', with at least one character in the range 'a' to 'z' or 'A' to 'Z'.

    The following extensions are valid:
    "name.txt"
    "name.hello33"
    "name.a$$$$$"
    "name.NoteToMyself"
    The following extensions are not valid:
    "name.#txt"
    "name.txt copy"
    "name.1.0.3"
    "name.extensiontoolong"
    "name."

Claimed Extensions

    In Mac OS X, an extension may be valid and still not be claimed. An extension is claimed if at least one application has specified in its property list that it claims the extension. Only valid extensions can be claimed, and more than one application can claim an extension. When this happens, some files may not open in the expected (or desired) application.

    Important Note for Users of Mac OS X:
    As you must already know by now, changing a file's extension can cause the file to open in a different application in Mac OS X. In some cases, the Finder may not be able to determine an application with which to open a file if the file has an invalid extension or unclaimed extension. It's difficult to state exactly when this will be the case since Mac OS X considers several factors when choosing the application to open a file.

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