How does OS X associate file extensions with apps?

Discussion in 'Mac Programming' started by ArtOfWarfare, Jun 3, 2013.

  1. ArtOfWarfare macrumors G3


    Nov 26, 2007
    I'd like to make a small utility that makes it a breeze to associate hundreds of file extensions with a single app and to use a single quick look plugin all at once.

    My understanding is that there's a property defined in each app bundle's info.plist that says which file extensions it associates with, but then the contents of this property are copied to some master file somewhere from time to time.

    Does anyone know where that master file is?

    Or should I just modify the chosen app's info.plist?
  2. robbieduncan Moderator emeritus


    Jul 24, 2002
    1) Do not ever modify another apps Info.plist: this will break code signing which will prevent them running if signed.

    2) Launch Services maintains the database of associations.
  3. ArtOfWarfare, Jun 4, 2013
    Last edited: Jun 4, 2013

    ArtOfWarfare thread starter macrumors G3


    Nov 26, 2007
    Okay, that makes sense. The things I was reading that said it was a viable option were all from between 2003 and 2005, so I was checking if anyone had newer answers.

    I'll look at the link you shared - thanks!

    Edit: Quick related question:

    Does a file itself contain its own MIME type - or is that some kind of meta data from outside the file?
  4. Madd the Sane, Jun 12, 2013
    Last edited: Jun 12, 2013

    Madd the Sane macrumors 6502a

    Madd the Sane

    Nov 8, 2010
    To my knowledge, Launch Services can map MIME data to document definitions and/or a UTI. But as far as storing MIME type for a file, it doesn't do that.
  5. ratsg macrumors 6502

    Dec 6, 2010
    all of the above is accurate, at least as far as I know.

    Just to be complete, my rule of thumb for app associations is:

    dos/windows - file name extension.
    older Mac OS - resource fork
    Unix - pulled from /etc/magic

    I just looked in my 10.6.x server, and I don't see a /etc/magic file. I *thought* a magic file was there, maybe in older versions and Apple has more recently pulled it.

    Again, on my 10.6.x Server, I do have (2) magic files, one in each of these directories.


    Hope this is of some value to you.
  6. Madd the Sane macrumors 6502a

    Madd the Sane

    Nov 8, 2010
    I think the Spotlight service also uses the magic file
  7. Meir David macrumors newbie

    Jun 13, 2013
    hi dude

    When a user drags an application into the Applications folder, the system should register it with Launch Services automatically.

    See Application Registration in the Launch Services Guide.
  8. kaydell.leavitt macrumors regular

    Apr 19, 2010
    Some History of File Types on the Mac

    Back in Classic Mac OS, each document has an OSType which was a 4 character file type such as "TEXT" for a text file, and file name extensions were just a part of the file name and had no special meaning.

    When OS X came out, it was Unix so, file name extensions came into use with Mac OS X. They are hidden by default in the Finder, but they are visible in and they are visible in if you go into Finder > Preferences > Advanced Preferences > Show all filename extensions and check this check box.

    OS X still has the OSType in application bundles. Application bundles are really just folders with the .app extension. You can open them up in Finder by right-clicking on them and choosing "Show Package Contents" from the context menu. If you look around, you can find a file called "PkgInfo", it contains two OSTypes one is the document type which is "AAPL" which stands for "application" and the second OSType is what's called the "creator signature".

    This OSType stuff is a carry-over from the Mac Classic OS era, but it is still around.

    I'll echo what somebody else said. Don't change any applications in any way!!!.

    In the Classic Mac OS, when a user opened a document from the Finder, the Finder would look at the creator signature to see which document created it and would open that application.

    Back in the Classic era, each file was potentially two files in one. 1) a data fork, and 2) a resource fork, and there were some file attributes which were only used on Macs and you could lose the resource fork and Apple's file attributes if you transferred the file to a computer that wasn't a Mac.

    Nowadays, OS X apps *can* create documents with a resource fork, but they usually don't or if they do, they don't store anything critical in the resource fork.

    Somebody else mentioned "Launch Services". This seems like a "clean" way to go, to go through one of Apple's API functions:


    In addition to OSType and filename extension, there is another way to type files that's used now called Uniform Type Identifiers (UTI). Here is a link to that sort of thing in Wikipedia:

    So in the history of Mac OS, a text file could be typed in the following manners:

    1. OSType of "TEXT"
    2. file name extension of ".txt", or ".java", or ".c" or ".cpp", etc, etc, etc.
    3. A UTI of "public.text"

    Then there's the MIME types which are more specific than just saying that something is text. For example:




    So, a file can be typed so many different ways, it can be confusing. When you figure it all out, let me know.

    -- Kaydell

    Whatever you do, don't write to one of Apple's files that is somewhere or change an application in any way.

    I could really use an application like the one that you're writing to change all text files with whatever extension to open up with BBEdit.

    Though, be careful not to annoy people by changing file type associations behind people's backs you want to put the user in charge.

    If you need a beta-user or if you have any more questions, you can send me an email. I'd love to hear from you.
  9. MeFromHere macrumors 6502

    Oct 11, 2012
    Why doesn't the functionality in Finder's "Get Info" window suffice? There's an "Open with" popdown menu and a "Change All..." button.
  10. IrishMafia macrumors newbie

    Dec 29, 2011
    Get Info Works

    As was suggested earlier, Get Info works! I loaded a new program that took my file association for .m4v away from Quicktime. I selected a movie file and changed the "Open With" back to Quicktime along with "Change All.."

    It just works. :)
  11. ArtOfWarfare thread starter macrumors G3


    Nov 26, 2007
    Yes, but I was asking about making this change with several file types all at once.
  12. jlettvin macrumors newbie

    Mar 2, 2014
    I agree with the original thread starter

    The answers fail utterly to achieve his/her goal.
    I have the same problem, and same goal.
    I want VLC to be my universal movie player
    and never again see QuickTime
    open and corrupt my library of math lectures.
    I have unrecoverably lost half a dozen movies.

    A utility to meet my need would be useful
    if it made any/all new movie types I give it
    launch VLC instead of QuickTime or, less comforting,
    if I could give it a list of all known movie extensions
    and associate VLC with all those extensions.

    Anything else leaves me at risk of destroying
    more of my precious lecture movies.

    So, I second the thread starter's request.
    I was unable to grasp enough of the Mac ecosystem
    to do it myself. Maybe in time...
    But the learning curve for this linux heavy is steep.

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