feat!: stricter validation of image links (check if the file is an image)

BREAKING CHANGE: links like ![Image](./not-an-image.txt) will now report
an error

package-lock.json

@@ -9,7 +9,8 @@
       "version": "0.0.0-development",
       "license": "MIT",
       "dependencies": {
-        "markdown-it": "14.1.0"
+        "markdown-it": "14.1.0",
+        "mime": "4.1.0"
       },
       "devDependencies": {
         "@types/markdown-it": "14.1.2",
@@ -3637,7 +3638,6 @@
       "version": "4.1.0",
       "resolved": "https://registry.npmjs.org/mime/-/mime-4.1.0.tgz",
       "integrity": "sha512-X5ju04+cAzsojXKes0B/S4tcYtFAJ6tTMuSPBEn9CPGlrWr8Fiw7qYeLT0XyH80HSoAoqWCaz+MWKh22P7G1cw==",
-      "dev": true,
       "funding": [
         "https://github.com/sponsors/broofa"
       ],

package.json

@@ -36,7 +36,8 @@
     "release": "semantic-release"
   },
   "dependencies": {
-    "markdown-it": "14.1.0"
+    "markdown-it": "14.1.0",
+    "mime": "4.1.0"
   },
   "devDependencies": {
     "@types/markdown-it": "14.1.2",

src/index.js

@@ -1,5 +1,6 @@
-import { pathToFileURL } from "node:url"
+import { fileURLToPath, pathToFileURL } from "node:url"
 import fs from "node:fs"
+import mime from "mime"
 
 import { filterTokens } from "./markdownlint-rule-helpers/helpers.js"
 import {
@@ -69,7 +70,7 @@ const relativeLinksRule = {
           url = new URL(hrefSrc, pathToFileURL(params.name))
         }
 
-        if (url.protocol !== "file:") {
+        if (url.protocol !== "file:" && type !== "image") {
           continue
         }
 
@@ -83,6 +84,15 @@ const relativeLinksRule = {
           continue
         }
 
+        const mimeType = mime.getType(fileURLToPath(url))
+        if (type === "image" && (mimeType == null || !mimeType.startsWith("image/"))) {
+          onError({
+            lineNumber,
+            detail: `${detail} should be an image`,
+          })
+          continue
+        }
+
         if (url.hash.length <= 0) {
           if (hrefSrc.includes("#")) {
             if (type === "image") {

test/fixtures/invalid/invalid-image.md

@@ -0,0 +1,5 @@
+# Invalid
+
+![Image](../not-an-image.txt)
+
+![Image 2](mailto:not-an-image@pictures.com)

test/fixtures/not-an-image.txt

@@ -0,0 +1 @@
+not an image

test/index.test.js

@@ -147,6 +147,14 @@ test("ensure the rule validates correctly", async (t) => {
           },
         },
       },
+      {
+        name: "should be invalid with file with an image link that is not an image",
+        fixturePath: "test/fixtures/invalid/invalid-image.md",
+        errors: [
+          '"../not-an-image.txt" should be an image',
+          '"mailto:not-an-image@pictures.com" should exist in the file system',
+        ],
+      },
     ]
 
     for (const { name, fixturePath, errors, config = defaultConfig } of testCases) {