I'm guessing your asking for Construct 3? Sometimes we get people asking questions about Construct 2 on the Construct 3 forums.
.capx is the format for Construct 2 projects. Construct 3 uses
.c3p instead. C3 is capable of reading
.capx projects for backwards compatibility but it isn't possible to save as a
.capx with C3.
There is a guide on saving files with C3 here.