Create a variable, Fname (or whatever), that is scoped to the Package. Set the property, EvaluateAsExpression to True.
Set the Expression to be (and I may need help here but there are plenty of examples of Date Expressions in the forum): "My.Filename." + (DT_STR, 4, 1252) YEAR(@[System::StartTime])
+ RIGHT("0" + (DT_STR, 2, 1252) MONTH(@[System::StartTime]), 2)
+ RIGHT("0" + (DT_STR, 2, 1252) DAY(@[System::StartTime]), 2)
You can tweek that to use GetDate() instead of the System::StartTime, again look for examples here.. I believe you need to use DateAdd("Day", -1, GetDate()) instead of GetDate() -1, and you may need the "" around "Day" for use in SSIS.
Once you have the variable correctly defined:
Create an FTP Task and under the File Transfer option set the IsRemotePathVariable to True.
Set the RemoteVariable to the variable just defined above. It should appear as User::Fname, using the name above.
Fill out the rest of the appropriate options for the Ftp Task.
To follow up on a previous recommendation, I would download the file to a local folder, import that file into a staging table - stgDailyImport (or whatever), truncating the table at the beginning of the operation and once the data is imported into that table, archive the txt file then continue with your normal processing, referencing the stgDailyImport.
Hope it helps..