Hi Gil,
As discussed
here, the TFW file format does not contain timing data, so no, I don't think there is a way for you to define the sampling rate in the file.
For a csv, the AFG ignores timing data in the file (all it cares about are the sample values), so you have to manually set the sample rate. In the advanced menu this can be done directly through the menu you can pull up at the bottom. In basic mode, there's no explicit way to do this, you just have to calculate the correct period from your number of samples in the file and desired sample rate.