diff --git a/src/__tests__/getFileMode.js b/src/__tests__/getFileMode.js
new file mode 100644
index 0000000..91bcae6
--- /dev/null
+++ b/src/__tests__/getFileMode.js
@@ -0,0 +1,51 @@
+"use strict"
+
+const getFileMode = require("../getFileMode")
+
+test("undefined if filename is empty", () => {
+  expect(
+    getFileMode({ htmlExtensions: [], xmlExtensions: [] }, undefined)
+  ).toBe(undefined)
+})
+
+test("'html' if filename matches an HTML extension", () => {
+  expect(
+    getFileMode({ htmlExtensions: ["html"], xmlExtensions: [] }, "foo.html")
+  ).toBe("html")
+})
+
+test("'html' if filename matches an HTML and an XML extension", () => {
+  expect(
+    getFileMode(
+      { htmlExtensions: ["html"], xmlExtensions: ["html"] },
+      "foo.html"
+    )
+  ).toBe("html")
+})
+
+test("'xml' if filename matches an XML extension", () => {
+  expect(
+    getFileMode({ htmlExtensions: ["html"], xmlExtensions: ["xml"] }, "foo.xml")
+  ).toBe("xml")
+})
+
+test("undefined if filename ends with extension without dot", () => {
+  expect(
+    getFileMode({ htmlExtensions: [], xmlExtensions: ["xml"] }, "fooxml")
+  ).toBe(undefined)
+})
+
+test("works with extensions starting with a dot", () => {
+  expect(
+    getFileMode({ htmlExtensions: [".html"], xmlExtensions: [] }, "foo.html")
+  ).toBe("html")
+})
+
+test("works with extensions containgin a dot", () => {
+  expect(
+    getFileMode(
+      { htmlExtensions: [".html.in"], xmlExtensions: [] },
+      "foo.html.in"
+    )
+  ).toBe("html")
+})
diff --git a/src/getFileMode.js b/src/getFileMode.js
new file mode 100644
index 0000000..6219955
--- /dev/null
+++ b/src/getFileMode.js
@@ -0,0 +1,25 @@
+"use strict"
+
+module.exports = function getMode(pluginSettings, filenameOrOptions) {
+  const filename =
+    typeof filenameOrOptions === "object"
+      ? filenameOrOptions.filename
+      : filenameOrOptions
+
+  if (!filename) {
+    return
+  }
+  if (pluginSettings.htmlExtensions.some(hasExtension)) {
+    return "html"
+  }
+  if (pluginSettings.xmlExtensions.some(hasExtension)) {
+    return "xml"
+  }
+
+  function hasExtension(extension) {
+    if (!extension.startsWith(".")) {
+      extension = `.${extension}`
+    }
+    return filename.endsWith(extension)
+  }
+}
diff --git a/src/index.js b/src/index.js
index db44d4f..1881dfc 100644
--- a/src/index.js
+++ b/src/index.js
@@ -5,6 +5,7 @@ const extract = require("./extract")
 const utils = require("./utils")
 const splatSet = utils.splatSet
 const getSettings = require("./settings").getSettings
+const getFileMode = require("./getFileMode")
 
 const PREPARE_RULE_NAME = "__eslint-plugin-html-prepare"
 const LINTER_ISPATCHED_PROPERTY_NAME =
@@ -108,21 +109,6 @@ In the report, please include *all* those informations:
   }
 }
 
-function getMode(pluginSettings, filenameOrOptions) {
-  const filename =
-    typeof filenameOrOptions === "object"
-      ? filenameOrOptions.filename
-      : filenameOrOptions
-  const extension = path.extname(filename || "")
-
-  if (pluginSettings.htmlExtensions.indexOf(extension) >= 0) {
-    return "html"
-  }
-  if (pluginSettings.xmlExtensions.indexOf(extension) >= 0) {
-    return "xml"
-  }
-}
-
 function patch(Linter) {
   const verifyMethodName = Linter.prototype._verifyWithoutProcessors
     ? "_verifyWithoutProcessors" // ESLint 6+
@@ -145,7 +131,7 @@ function patch(Linter) {
     }
 
     const pluginSettings = getSettings(config.settings || {})
-    const mode = getMode(pluginSettings, filenameOrOptions)
+    const mode = getFileMode(pluginSettings, filenameOrOptions)
 
     if (!mode || typeof textOrSourceCode !== "string") {
       return verify.call(