QuickPlan for macOS supports to creata a new project by importing from an OPML file. Watch the video

But very specially, we may get the error message “The document ‘***.opml’ could not be opened. QuickPlan cannot open files of this type” when opening an OPML file.

The error message shows that the macOS does not support to send OPML file to QuickPlan app as another app registered “opml” file extension for another purpose (which prevents other app to open the same type of file). As QuickPlan has no permission to change the macOS setting (QuickPlan is running in the sandbox environment with limited permission), this is out of the control of QuickPlan.

I want to suggest if you can convert the OPML file using QuickPlan for iPad, then sync to Mac via iCloud Drive. There is no direct solution to fix it as it’s out of the control of an app.

macOS controls how an app to open a file:

• When installing an app, the app tells the MacOS that a file extension is being supported (for this case, QuickPlan requires to support .opml file extension). macOS will add the app to the file menu list of the related file extension.
• Then when a user opens a file with the file extension, macOS will check the registration information and send the file to the app to open.

In your case, something wrong with the file extension registration information, macOS does not send the file to QuickPlan by replacing it with the error message. Apps downloaded from the App Store have just limited permission, which can only register the file extensions to macOS, but no way to control the macOS directly.

We can check the apps registered by macOS for a file type:

• Right-click on a file with .opml file extension
• Click “Get Info” menu
• In the file info dialog, please check the “Open with” dropdown list. The default app may prevent other app to open the files with the same file extension.