Is the issue isolated to Finale documents, or do you get a generic Program icon on all documents ?

It seems to be limited to Finale documents. PDFs, graphics and rtf, pages and docx text files all display as expected.

Do a Safe Boot by starting with the Shift key held down. Log in when you get to your Users screen. After you see that your desktop icons have been rebuilt, shut down and reboot normally. Safe Boot takes awhile on an SSD and feels like forever on an HDD. Be patient, it will complete.

This sets your caches to default and rebuilds your paths. In the old days, this was called Rebuild Desktop and done the same way. If you have generic Alias icons, this fixes that, too.

I'm not certain that is your issue but it may be and it is the first thing to try. I have done over 50 High Sierra and Mojave installs and every one showed a few generic aliases, the most visible symptom of corrupt pathways. Safe Boot is now part of that routine.

Thanks, Mike. I tried a safe reboot but it didn't help. What I don't understand is that one of the QuickLook folders (root library) does have qlgenerator files in it, just not for Finale. These folders have the same contents on both computers but I get Finale file previews on one computer and generic Finale icons on the other. I guess I'll try posting this on the other forum and also submitting it to MM.

MM wrote back to me and said that the qlgenerator hasn't yet been updated to 64 bit. The reason it works on the MacPro is that I have an earlier version of Finale still installed. Interestingly, copying the qlgenerator file into the Finale 26 program file didn't help, nor did copying the qlgenerator file to the QuickLook folder in the root library. Oh well.

Based on what you just posted, I know how to make it work. The question is, How badly do you want this?

Read this first. It’s not the way to do it but it is one of the tools you use in the process. Besides saving a lot of time, you will be able to completely restore to an earlier state in a few minutes if you don’t like the results.

http://www.motunation.com/forum/viewtopic.php?f=4&t=64689

Let us know and I will write a step by step.

Thanks, Mike, that's indeed an ingenious way of creating a snapshot, but I don't see how it helps my situation. The laptop doesn't produce QuickLook views of Finale files because it doesn't have an earlier version of Finale (including a qlgenerator file) on the disk. I still don't understand why the QuickLook file which is buried in the Finale 25 program file doesn't work when moved to either the Finale 26 program file or the QuickLook folder in the root library. Sure, the qlgenerator file is still 32 bits, but Finale 25 was already 64 bits, so what difference does that make?

I didn't explain why I showed you that link. If you don't like the results of the process or it doesn't work right, you can use APFS Snapshots to completely restore your Mac to an earlier state like to a few minutes before you began. It takes a few minutes unlike a complete Time Machine Restore — this can take hours.

1) The first thing I would try is to install 25, authorize it, then reinstall 26. Although they can exist side by side, they share and modify some system files. In an ideal world, this is the fix.

2) If unsuccessful, the next thing I would try is to bring over 25 and 26 from your other machine using Migration Assistant (in your Utilities Folder).

2a) If you have the same Apps installed on both machines, you can import your entire Applications folder this way. Do not copy them over. Migration Assistant should move over the Support files as well. 

3) You can pull the entire contents of your other Mac over using Migration Assistant—what I meant by, How badly do you want this?

I will presume that you know how to restore individual apps and files in Time Machine. You will find this process the same but a good deal faster if they are sitting in Snapshots. If old enough, you'll have to grab them from Time Machine which is slower. The process is seamless, it's the speed that's quicker if still residing in APFS. Here's one of the wicked cool features: you can un-delete files since the copy in Snapshots is at most a few minutes old; sometimes just seconds. Time Machine backs up every hour to an external disk. You can use this to restore your Documents or Application folder or anything you want, really.

Worst case: You've played around and it just won't behave — or it does and you just don't like the result. 

Shut down, boot holding Command r, restore from your Drive icon instead of a Time Machine volume. Select a time and hit OK. It takes less than a minute on my clunky old 2010 iMac. Your system is restored to the exact state it was before you started.

APFS Snapshots for a system restore works only if the Mac sees one OS on the system. Partitioned drives with multiple OS is a no-go. I have external test drives with operating systems that must be disconnected before it works. Partial/single file/folder restores are done by entering Time Machine the old way—they're just faster now. 

This has saved my butt a couple of times. A new software install really screws up another app and turns a project into a disaster? Go back to a minute before the install. How I know this isn't important but it was great to know I could go back in time without a DeLorean in minutes instead of many hours.

I thought I'd follow up on this. I just noticed that QuickLook is now working on my laptop without my having to install Finale 25 (an action suggested by MM). I copied the qlgenerator file from within the Finale 25 application to the same spot in the Finale 26 application package, with a link to it in my root library, although I'm not sure if that latter action was necessary. At least one restart was required; perhaps the resets caused by an interim system update helped, as well.