MacOS: How to change the default app for a file

Back in the pre-Mac OS X and macOS days, Apple’s System 9 and
earlier relied on hidden metadata to associate files with apps.
File extensions, those bits of text that follow a period at the
end of a file (like .doc, .html, or .jpg) were optional,
although often used for compatibility with other platforms and
with web. On the web, file extensions are effectively mandatory
so a browser knows how to handle a file appropriately.

Macworld reader Rick would like to monkey with that. He has a
number of HTML templates, but to differentiate them from his
production .html files, he puts the suffix .tt on them instead.
Browsers don’t recognize these files by default. There’s a way
to force an association between a file type and an application,
but that application still has to recognize the extension.

If you have an extension that’s simply not mapping correctly,
you can follow these steps:

  1. Select the file in the Finder and choose File >
    Get Info.
  2. In the Open With section, if there’s an
    appropriate app in the list, you can select it and click
    Change All and confirm, and now all files with that
    extension open in that app. You can stop here. But if the app
    you want doesn’t appear in the list, select Other.
  3. Choose the app from the list that shows. In the
    Enable pop-up menu, you can choose All
    Applications
    , and it will let you pick any app. Check the
    Always Open With box to force an association.
  4. Click Add.
mac911 assign apps to file types
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While you can assign apps to file types, the apps may
refuse to open them.

If you’re using, for example, .tt as your HTML template
extensions like Rick, you could go through steps 1 to 4, and
pick Safari as the app to open .tt files. The trouble is that
Safari doesn’t know that a .tt file contains HTML.

In the olden days, when everything to do with the web was more
in a state of flux, you could modify and add content mappings,
usually in the form used by MIME, a decades-old method of
associating actions and formats with file extensions. (You’ll
see MIME mentioned explicitly in email programs’ headers. Some
kinds of documents also embed MIME information into their
headers, so software can read a few characters of the file to
figure out what it is.)

mac911 tags editor macos
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The Tags editor lets you create your own, which you can
assign to files to sort and find them.

Unfortunately, there’s no way I can find to change file
associations in Safari or Chrome. Firefox exposes more of this
mapping information, but you can’t add new file types.

Might I suggest instead using macOS’s Tags feature? In the
Finder, select Finder > Preferences and
click the Tags icon. You can add an HTML Templates tag
and assign that to all your templates. Then you can use a Smart
Folder to gather them together, or use various Arrange By/Sort
By options to group by tags.

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