On 2017 Dec 14, Tim Streater wrote
(in t):

In , Your Name
wrote:

I don't know about Windows, but Mac OS X can have at least four letter
extensions (.tiff, .jpeg, .html for example).

It can be as long as you like, I suspect. Xojo uses
.xojo_binary_project, for example.

it can be as long as you like, so long as the filename, dot, and extension
fit into 255 characters. I don’t see this as a serious limitation.

hmmm...
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMONPQRSTUVWX YZ0123456789abcdefghijklmnop
qrstuvwxyzABCDEFGHIJKLMONPQRSTUVWXYZ0123456789abcd efghijklmnopqrstuvwxyzABCDEF
GHIJKLMONPQRSTUVWXYZ0123456789abcdefghijklmnopqrst uvwxyzABCDEFGHIJKLMONPQRSTUV
WXYZ0123456789.123456 is 255 characters. That’s one hell of a filename.