I would not even try to filter a flat file. I am not surprised it does not work. A flat file must be read sequentially. The filter option is offered, but on the basis that the user understands that a flat file cannot be "selected" from since it can only be read sequentially. So while DocuWare is allowing you to create a filter on the file, the error message is correct. If you want to filter this data you need to import the file in to a database table where a query can be filtered.
I guess the problem is that DocuWare is allowing a filter to be created against a flat file but given that it not possible it is assumed that it will not be attempted.
And the flat file should be the file that is read from. It should be the "iterated" file. Meaning you should read the file line-by-line and select the appropriate record from DocuWare - which can be filtered. If your iterated source is DocuWare, then for every record selected from the DocuWare file cabinet, you will read every record from the flat file until a match is found for every record from the DocuWare table.
Senior Director Support Americas