The OS indeed does care about how you name your files. As you mentioned on linux based systems filenames starting with a dot are hidden for example
But on Windows such a filename wouldn't be allowed because it needs at least a basefilename. The file extension is the indicator with which program the file should be opened. If the file extension is missing Windows asks every time what program should be used to open the file.
On linux based systems files don't need a file extension because the system reads the file header and decides then what program should be used to open it.
Not to mention that some special characters are not allowed in filenames, depending on the OS.
I still believe that a
.txt filename for example in this puzzle shouldn't be identified as
text/plain MIME-Type because of the reasons I metioned above. That is not how it works. If somebody can convince me that this works, then I said nothing.