If the filename has several times EXT_SHOULD in its filename if may have been a problem.
If the filename doesn't contain EXT_SHOULD in its filename, it is a problem as we would
overwrite the input file.
If EXT_SHOULD is a prefix of the actual extension, the log and tap files won't have
a .log or .tap extension.

It's probably more flexible to let the user choose his/her own extension, and we deal with that.