Registered mime-types and default handlers on OS X

Which application is the default handler for file type .blah on OS X? Turns out this is actually a pretty complicated question.

Launch Services keeps a database, into which applications can register themselves as mime handlers, using CFBundleDocumentTypes and CFBundleURLTypes in Info.plist in their application Contents, which are parsed periodically (not sure when). Here is part of Safari's:

$defaults read /Applications/Safari.app/Contents/Info.plist 
{
    "Application-Group" = "dot-mac";
    BuildMachineOSBuild = 12A251;
    CFBundleDevelopmentRegion = English;
    CFBundleDocumentTypes =     (
                {
            CFBundleTypeExtensions =             (
                css
            );
            CFBundleTypeIconFile = "document.icns";
            CFBundleTypeMIMETypes =             (
                "text/css"
            );
            CFBundleTypeName = "CSS style sheet";
            CFBundleTypeRole = Viewer;
            NSDocumentClass = BrowserDocument;
        },
                {
            CFBundleTypeExtensions =             (
                pdf
            );
            CFBundleTypeIconFile = "document.icns";
            CFBundleTypeMIMETypes =             (
                "application/pdf"
            );
            CFBundleTypeName = "PDF document";
            CFBundleTypeRole = Viewer;
            NSDocumentClass = BrowserDocument;
        },
[snip]

Also as an aside, safari has a list of file types that it will automatically open with the registered mime handler because they are considered 'safe'. The list is stored in:

/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/System

Where:

• LSRiskCategorySafe: Safari will automatically open these files after download
• LSRiskCategoryNeutral: Not auto-opened, no warnings
• LSRiskCategoryUnsafeExecutable: Warning displayed when opened by the user.
• LSRiskCategoryMayContainUnsafeExecutable: e.g. zip files. This will trigger a warning if Safari can't determine that the contents are safe or neutral

Back to the LaunchServices database. You can use API calls to register a mime-type, and there is also a helper utility called lsregister that provides a simple interface to the database. This command will dump the contents of the database, there are also options to force a registration:
There is also an app (RCDefaultApp) that presents a more usable grapical frontend to the data.

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -dump | less

The user also has some control over mime-type registration, and that information is stored in:

$ defaults read ~/Library/Preferences/com.apple.LaunchServices
{
    LSHandlers =     (
                {
            LSHandlerRoleAll = "com.google.chrome";
            LSHandlerURLScheme = http;
        },
                {
            LSHandlerRoleAll = "com.google.chrome";
            LSHandlerURLScheme = https;
        },
                {
            LSHandlerContentType = "public.html";
            LSHandlerRoleViewer = "com.google.chrome";
        },
                {
            LSHandlerContentType = "public.url";
            LSHandlerRoleViewer = "com.google.chrome";
        }
    );
}

Reading the output of the database query and the plist above, it is difficult to determine which handler will fire for certain files where multiple handlers are registered. Apple provides some documentation, about how the mime handler is chosen for files and URLs in the case of multiple handers, but the flow chart ends with:

If two or more candidate applications remain after all of the foregoing criteria have been applied, Launch Services chooses one of the remaining applications in an unspecified manner.

The quickest way to check is to use 'open', which according to the man page:

opens a file (or a directory or URL), just as if you had double-clicked the file's icon.

So, creating a dummy css file and running open like this should pop up safari:

open blah.css

Find out a file's mime type with:

$ mdls -name kMDItemContentType -name kMDItemContentTypeTree test.dmg 
kMDItemContentType     = "com.apple.disk-image-udif"
kMDItemContentTypeTree = (
    "com.apple.disk-image-udif",
    "com.apple.disk-image",
    "public.archive",
    "public.data",
    "public.item",
    "public.disk-image"
)