You will probably find help around here:
The first thing to know about the Attachments is that they are not Stored within the Database, but rather on a Folder.
The Attachment Record contains the File Name of the Attachment as it is Stored on the Disk after being attached.
Depending on how Long you have SalesLogix, you may see different Naming Conventions around, but the Statement Above remains true.
The Second thing to know, which is obvious from Number one is that losing Data from Attachments may have ocurred if the Drive was "cleaned" and files were removed, or if you have corruption in the Disk (I have seen cases where old attachment files don't open anymore). Sometimes it is due to Versions of the Program (e.g. Opening a 10 years old PDF with a new Version of FoxIt).
My point being that if your Attachment Table has 2,000 records, you should have 2,000 files on the Drive. It is possible that through the years files records have been deleted and files haven't (or the other way around).
Third point, Mail Merge Attachments are stored under a SubFolder on the Attachments folder. In that case the File Name will include the name of the Folder: "Mail Merge\!QU6J9A000001.doc".
Fourth, the Description field is typically the original name of the File. For instance, if I attach a file called "My Bank Statement.pdf", the Description will Default to "My Bank Statement" and the name will be set to "!U6J9A000001.pdf"
This means that you could generate a Friendly Name for each file based on the Description (watch out for illegal characters on this field e.g. "?" It is a valid character on the DB, but not on the file system), and associate it with the extension (read from the FileName field).
That said, your first task should be to reconcile the Attachments Table vs the Attachments folder and determine which Records are Orphan (no file). You may also determine which files are no longer linked to the DB and put them aside.
Hope this helps you understand a bit better how the Attachment table works, and if you need assistance, there are certain a few of us on this board that may be able to work with you to help you achieve your Migration.