Short-term remedy
In many cases you don’t want to change the default app for opening a file, but rather select an alternative program as an exception. To do this, select the file using a secondary click (right click, two-finger click or -click) to open the context menu. Move the mouse pointer to “Open with…” to see a list of all compatible programs. Alternatively, you can also open the file menu with the file selected – the same submenu can be found there. In this case, macOS does not remember that the file should be opened with this program.
You can spontaneously select a different program using the storage and context menus.
File specific: “Information”
If you want to open this file permanently with another program, select “Information” from the context or file menu (keyboard shortcut +I). An additional window immediately appears with lots of details about this document. In the “Open with” entry (possibly opened by clicking on “>” to the left of it), the Finder shows which program starts when you double-click on the file. In the drop-down menu you can choose another program to open this file – then you can close the window. In this case, macOS remembers your preference for this one file and will open it in the future when you double-click the document. If the program you want does not appear in the list, click on “Other…” at the bottom and select the app yourself. If the program remains grayed out, select “All Programs” from the Activate drop-down menu.
The Finder’s information dialog allows you to set a different editor for a file.
Change for all files
You may also want to decide once and for all which program starts when you double-click a file with the same extension. In this case, proceed in the same way as just described: Open the information dialog and select your favorite program. In this case, however, click on “Change all…” and confirm your choice in the dialog that appears. All files with the same extension will then open this program by default.
By additionally clicking on “Change all…” you set the new editor as the default for the file type.
In the background: extended attributes and UTIs
How the Finder decides what to open a file with is determined by a multi-stage query, as Howard Oakley explains in an article: File-specific settings are stored in a file attribute, more precisely: in “com.apple.LaunchServices.OpenWith”. The Uniform Type Identifier (UTI) determines which programs are assigned to a file extension.
Fix problems
Oakley recommends two of its free programs for analysis: UTIutility helps with reading and changing the uniform type identifiers. In his experience, if a file is assigned to the correct program but macOS does not want to open it, this is due to a quarantine attribute that has been set incorrectly. He has the Quarant2 tool ready for such cases.
Special case Quick Look
macOS uses a similar, even more difficult to understand system when it comes to deciding which app creates the overview of a file. This happens when you press the spacebar after selecting a file – in this case a file preview appears without having to start a program. Developer Brett Terpstra recently stumbled upon this after launching the Markdown overview extension “Marked QL” in the App Store. In order to track down conflicts between Quick Look extensions, he quickly programmed Quick Look Doctor. Users can decide for themselves which overview extension is used for a specific file type.
Quick Look Doctor reveals which overview extensions are available for a file.