diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 9625dbe..57b5788 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -90,4 +90,11 @@ cargo clippy
 
 However, since `clippy` produces a compilation of the project, I wouldn't
 recommend running every time you save a file, since it would slow down
-your productivity.
\ No newline at end of file
+your productivity.
+
+#### Testing the code
+
+If your PR is addressing a new feature, then writing unit tests would be
+necessary. The unit test cases should both test private and public methods,
+so you will place them inside the same file as your code, in a `tests` module
+inside that file.
\ No newline at end of file
diff --git a/src/html_page.rs b/src/html_page.rs
index 796fe3c..804f10d 100644
--- a/src/html_page.rs
+++ b/src/html_page.rs
@@ -189,3 +189,31 @@ impl HtmlPage {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Result as IoResult;
+
+    #[test]
+    fn file_name_refers_to_text_file() -> IoResult<()> {
+        let file_name = "file.txt";
+        HtmlPage::is_path_to_text_file(&file_name)
+    }
+
+    #[test]
+    fn file_name_does_not_refer_to_text_file() {
+        let file_name = "file.html";
+        let error = HtmlPage::is_path_to_text_file(&file_name);
+
+        assert!(error.is_err());
+    }
+
+    #[test]
+    fn file_name_does_not_have_extension() {
+        let file_name = "file";
+        let error = HtmlPage::is_path_to_text_file(&file_name);
+
+        assert!(error.is_err());
+    }
+}