Assuming the data driven subscription is based on something like below.
CREATE TABLE dbo.Subscribers(
1. Keep the format as CSV to preserve the output format. The TXT value is unrecognised so will not succeed.
2. When asked for the query calculate the filename of the attachment
SELECT EmailRecipient, Format ParameterValueX, 'Data.TXT' as Filename
3. During the data driven subscription wizard set :
- the option for format to the Format column or hard-coded to CSV
- the option for file extension to false (i.e. you do not want .CSV added to the end of the attachments filename)
- the option for file name should be the filename column from the query (not .TXT showing at the end