How to remove the warning is written very clear and with wonderfull examples in the warning itself.
In your particular case, it should look like this:
defp filter_content_type(list, acc \\ %{})
defp filter_content_type([h|t], acc) when is_map(acc) do
# Stuff
end
defp filter_content_type([], acc) when is_map(acc) do
acc
end
