A couple of comments:
1. If a file has no file extension the OS will sometimes take a guess at the file type, which might explain the Font Book experience.
2. If you have created files that should be xl files, you could try manually adding the file extensions in Finder. Finder will post dialog saying this is unwise (which it is, but it can be done), but just ignore this and do it anyway (I'd suggest using .xls rather than .xlsx as it's more generic). If the file subsequently opens with Excel then all is well. If it doesn't, then remove the extension you just added—you'll get the same warning again, which you again ignore.
3. A big tick to your learned lesson about coding export files—don't forget the file extension!
4. Hiding file extensions is a Finder preference (see screenshot), and I suggest you always show them.
1. Interesting bit is how Filemaker is creating slightly different files when exported from various version on the same OS.
2. Could do - but not client friendly.
3. Knowledge is power.
4. Knew that but don't care for the look.