The reason is that dbFront can't determine what the file extension should be.
When you configure an Object Field type you have the option of specifying a secondary "Filename Field" that will be used when dbFront needs the attachment filename.
dbFront uses the file extension as found in the "Filename Field" and the actual attachment contents to determine what the real file attachment type is. For security reasons, dbFront does not trust the attachment filename.
If the file attachment type and the file extension don't agree then dbFront returns [Unknown] as the filetype.
In addition, if the file type can't be determined from the contents and there is no "Filename Field" specified then dbFront will also return [Unknown].
For more details see: Object Fields