FolderItem.Type
From Xojo Documentation
Read-Only Property (As String )
If the FolderItem is not a folder, this property contains the file type of the FolderItem. If the FolderItem is a folder (macOS), it will return a matching file type if you have created one. The file types must be defined in your project. If not, this property will be empty.
Notes
This property only contains a type if you open a file for which there is a corresponding File Type Set.
Example
If the standard "text/plain" file type been added to a File Type Set, then this gets the type: