Hi Rg144,
An FXT file is required for every time-frame you wish to back-test in (eg. if you select “M5” in the as the Tester’s “Period” you will need the 5 minute FXT file). The FXT file contains the detailed tick data that will be used by the strategy tester.
Technically speaking, the contents of the FXT file between time-frames is exactly the same except for a small piece of information in the header which describes the time-frame. It is this piece of information that unfortunately disallows you from simply renaming FXT files so you can back-test in another time-frame.
Hope this answers your question.