filePath is an atom--not a variable:

7> is_atom(filePath).
true

In erlang, variables start with a capital letter. The only way erlang could match your function clause would be to call the function like this:

appender_server:start_link(filePath)

Here is an example:

-module(a).
-compile(export_all).

go(x) -> io:format("Got the atom: x~n");
go(y) -> io:format("Got the atom: y~n");
go(X) -> io:format("Got: ~w~n", [X]).

In the shell:

3> c(a).
a.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,a}

4> a:go(y).
Got the atom: y
ok

5> a:go(x).
Got the atom: x
ok

6> a:go(filePath). 
Got: filePath
ok

7> a:go([1, 2, 3]).
Got: [1,2,3]
ok

8>