We are aware of Workbooks users not able to filter on the column 'Import Type', this is due to some engineering work behind the scenes that disables the user to filter on this column in the conventional way. This can be particularly important when your business is running quite frequently Import.  

Please see the screenshot below:

However there is a workaround for this which replaces the use of alphabets on that filter and allows the user to lookup record types with their associated numeric values.