An example:

We have a directory, an actual file, and a symlink to a file and to a directory:

.
├── actual_directory/
├── actual_file
├── symlink_to_directory -> actual_directory
└── symlink_to_file -> actual_file

Then:

?- exists_file(actual_directory).
false.

?- exists_file(actual_file).
true.

?- exists_file(symlink_to_directory).
false.

?- exists_file(symlink_to_file).
true.

?- exists_file(yappadappadoowilmaaa).
false.

This seems to behave like the Perl -f file test flag, see:

https://perldoc.perl.org/functions/-X

$ perl -e 'if (-f "actual_directory") { print "YES\n" } else { print "NO\n" }'
NO

$ perl -e 'if (-f "symlink_to_directory") { print "YES\n" } else { print "NO\n" }'
NO

$ perl -e 'if (-f "symlink_to_file") { print "YES\n" } else { print "NO\n" }'
YES

$ perl -e 'if (-f "actual_file") { print "YES\n" } else { print "NO\n" }'
YES

$ perl -e 'if (-f "yappadappadoowilmaaa") { print "YES\n" } else { print "NO\n" }'
NO

Here is a radical idea: why not dump the C straightjacket and become more Perlish:

filetest(+Flag,+File)

would succeed or fail depending on whether the Perl flag Flag applies. That sounds like a powerful addition.