Allow an optional extension to be passed to nextfile

spec/nextfile_spec.lua

@@ -83,5 +83,13 @@ describe("nextfile", function()
 
 			assert.are.equal("/testdir/421za.txt", file)
 		end)
+
+		it("should use the provided file extension if one is given", function()
+			prevfile.prevfile = function() return "/testdir/900.txt" end
+
+			local file = nextfile.nextfile("/testdir", "yaml")
+
+			assert.are.equal("/testdir/901.yaml", file)
+		end)
 	end)
 end)

src/nextfile.lua

@@ -15,19 +15,28 @@ function nextfile.nextletter(letter)
 	return string.char(nextascii)
 end
 
-function nextfile.nextfile(directory)
+function nextfile.nextfile(directory, extension)
 	local previous = prevfile.prevfile(directory)
 	if previous == nil then return nil end
 
 	local dir, name, ext = nextfile.splitfilename(previous)
 
+	value = dir
 	if tonumber(name) == nil then
 		local lastletter = name:sub(#name)
 		local nextletter = nextfile.nextletter(lastletter)
-		return dir .. name:gsub(lastletter .. "$", nextletter) .. ext
+		value = value .. name:gsub(lastletter .. "$", nextletter)
+	else
+		value = value .. name + 1
 	end
 	
-	return dir .. name + 1 .. ext
+	if extension ~= nil then
+		value = value .. "." .. extension
+	else
+		value = value .. ext
+	end
+
+	return value
 end
 
 return nextfile