test(util): add file extension tests

tests/util_spec.lua

@@ -2,13 +2,28 @@ local util = require("img-clip.util")
 
 describe("util", function()
   it("can get the directory path from the file path", function()
-    -- construct test input
     local path_separator = package.config:sub(1, 1)
     local test_path = "path" .. path_separator .. "to" .. path_separator .. "file.txt"
     local expected_dir_path = "path" .. path_separator .. "to" .. path_separator
 
-    -- check if the correct directory path is returned
     local dir_path = util.get_dir_path_from_filepath(test_path)
     assert.equals(expected_dir_path, dir_path)
   end)
+
+  it("can add a file extension to a filename", function()
+    assert.equals("file.png", util.add_file_ext("file", "png"))
+    assert.equals("file.png", util.add_file_ext("file.png", "png"))
+    assert.equals("file.png", util.add_file_ext("file.jpg", "png"))
+    assert.equals("file.png", util.add_file_ext("file.hello.world", "png"))
+    assert.equals("file.jpg", util.add_file_ext("file", "jpg"))
+  end)
+
+  it("can add a file extension to a filepath", function()
+    local path_separator = package.config:sub(1, 1)
+    local test_filepath = "path" .. path_separator .. "to" .. path_separator .. "file"
+    local expected_filepath = "path" .. path_separator .. "to" .. path_separator .. "file.png"
+
+    local filepath = util.add_file_ext(test_filepath, "png")
+    assert.equals(expected_filepath, filepath)
+  end)
 end)