Maybe not that long, but it can be just about anything the developer wants. As Apple changed to appease the rest of the world, it stopped using what they called 'creator' codes along with the 'type' code. Now all we have are the 'type' codes. Back in the early days, there were only a few 'type' codes; text, html, etc. Then people created documents with lots more 'stuff' in the file, so they needed more specific 'types'; 'doc', 'php', 'exe', 'fm', etc. Obviously, two, three, and even four characters was not enough to cover the thousands of new programs and their proprietary files. Numbers (the spreadsheet app from Apple) uses... wait for it... '.numbers'! (Genius and unlimited imagination at work?
)
In actuality, however, many files are really nothing more than plain text! It's a dirty little secret that some developers don't want us to know. They obviously don't want us to stop using (paying for!) their program when we could open their documents with nothing more than a free app like TextEdit. Of course, opening a file is not the same as viewing the data in the file coherently or even usefully! The developer may be creating a text file, but that doesn't mean that we can understand how the characters should be displayed!
This whole exercise is prompted by the extremely bad reviews I saw about MacJournal. It would be a shame to lose most, if not all, the work you've put into creating your journal(s). As I say, I've never used the app, but its heritage of coming from a word processor make me hope that its files can be opened in another app. It might store all the textual data in a simple, plain text format. It might be impossible to gather all the links that may relate one entry to another, the dates/times may be stored as large hexadecimal numbers, etc. No one may know except the developer(s). Unfortunately, they may not be too helpful with these kinds of details. It appears, from the reviews, that the manner things are stored may have changed in the new version. Apparently making old 'journals' un-viewable in the new version!
Not exactly a way to win new sales, much less repeat customers.
Just because a file has a '.macjournal' suffix doesn't really mean it's not, in fact, just plain text. It just means that the OS has registered that suffix as belonging to the MacJournal app. Double-clicking one of those files will then cause the OS to open it in MacJournal.app instead of TextEdit, or anything else.
My main concern is that you make sure to have at least one backup of all your original journals from your old computer. If the new version of the app can and will use them, great! If not, you'll need to do a bit more investigation to see if they can be salvaged.
As for the length of suffixes, how about these:
ADIUMEMOTICON
ADIUMEMOTICONSET
ADIUMMENUBARICONS
ADIUMMESSAGESTYLE
ADIUMPLUGIN
ADIUMSCRIPTS
ADIUMSERVICEICONS
ADIUMSOUNDSET
ADIUMSTATUSICONS
Seems the 'Adium' folks like things to be self-explanatory!