Textopia, occasional referred to in its documentation as "Textopia-Toolbox", is a German interactive fiction authoring system created by "Oliver Berse" (presumably also the author of Floyd, although there is not any overlap between the formats) in 1999. Textopia has no compiler, interpreter, etc. in a strict sense; instead, it is implemented as a Pascal library. As such, Textopia source files and the resultant games have no special extensions or structure, beyond that of generic Pascal files.
It is probably possible to identify Pascal programs that use the Textopia library by their library imports, which is, in all the demos,
USES TSTRING,TIO,TMAIN, although presumably the order and other aspects of arrangement are not fixed. Beyond that, the strings
PGame are probably common enough that at least some of them will appear in all source files.