Yes, I am sure: for example, see the attached screenshot of the "Get Info" window for those files, which matches the "File Names" screenshot in my opening post (i.e. extension shown for the first three files and hidden for the fourth one).
Moreover, as mentioned, the very same files transclude perfectly through Marked 2 by using, in a transclusion file with the Marked 2 transclusion syntax, the very same file names and extensions I had used in the transclusion file I created to transclude through Composer.
For some unknown reason, I can not replicate the issue either: today, everything works just fine, even though I have not changed anything: I have not restarted Composer or the computer; since yesterday, I did quit a few applications, though (e.g., Marked 2, Tinderbox 8, DTPO/DT3 and SublimeText 3). I wondered whether any of those were somehow interfering, so I re-opened them all, but no: file transclusion through Composer still works perfectly... So I am not sure what to say, other than I am sorry for wasting your time. Thank you for bearing with me.