docs: clarify MIME type changes when migrating from Express 4 to 5 (#1903)

* docs: clarify MIME type changes when migrating from Express 4 to 5

* fix: removed trailing spaces

* fix: added router.params(fn) back

* docs: specify exact MIME type changes in Express 5 upgrade notes

---------

Co-authored-by: Sebastian Beltran <[email protected]>

Author: achaljhawar

en/guide/migrating-5.md

@@ -51,7 +51,7 @@ You can find the list of available codemods [here](https://github.com/expressjs/
   <li><a href="#req.param">req.param(name)</a></li>
   <li><a href="#res.json">res.json(obj, status)</a></li>
   <li><a href="#res.jsonp">res.jsonp(obj, status)</a></li>
-  <li><a href="#magic-redirect">res.redirect('back') and res.location('back')</a></li>  
+  <li><a href="#magic-redirect">res.redirect('back') and res.location('back')</a></li>
   <li><a href="#res.redirect">res.redirect(url, status)</a></li>
   <li><a href="#res.send.body">res.send(body, status)</a></li>
   <li><a href="#res.send.status">res.send(status)</a></li>
@@ -322,6 +322,15 @@ app.get('/user', (req, res) => {
 
 The `res.sendfile()` function has been replaced by a camel-cased version `res.sendFile()` in Express 5.
 
+**Note:** In Express 5, `res.sendFile()` uses the `mime-types` package for MIME type detection, which returns different Content-Type values than Express 4 for several common file types:
+
+- JavaScript files (.js): now "text/javascript" instead of "application/javascript"
+- JSON files (.json): now "application/json" instead of "text/json"
+- CSS files (.css): now "text/css" instead of "text/plain"
+- XML files (.xml): now "application/xml" instead of "text/xml"
+- Font files (.woff): now "font/woff" instead of "application/font-woff"
+- SVG files (.svg): now "image/svg+xml" instead of "application/svg+xml"
+
 {% include admonitions/note.html content=codemod-deprecated-signatures %}
 
 ```js
@@ -345,6 +354,15 @@ The `router.param(fn)` signature was used for modifying the behavior of the `rou
 In Express 5, `mime` is no longer an exported property of the `static` field.
 Use the [`mime-types` package](https://github.com/jshttp/mime-types) to work with MIME type values.
 
+**Important:** This change affects not only direct usage of `express.static.mime` but also other Express methods that rely on MIME type detection, such as `res.sendFile()`. The following MIME types have changed from Express 4:
+
+- JavaScript files (.js): now served as "text/javascript" instead of "application/javascript"
+- JSON files (.json): now served as "application/json" instead of "text/json"
+- CSS files (.css): now served as "text/css" instead of "text/plain"
+- HTML files (.html): now served as "text/html; charset=utf-8" instead of just "text/html"
+- XML files (.xml): now served as "application/xml" instead of "text/xml"
+- Font files (.woff): now served as "font/woff" instead of "application/font-woff"
+
 ```js
 // v4
 express.static.mime.lookup('json')
@@ -394,7 +412,7 @@ app.get('/*splat', async (req, res) => {
 ```
 
 {% capture note_wildcard %}
-`*splat` matches any path without the root path. If you need to match the root path as well `/`, you can use `/{*splat}`, wrapping the wildcard in braces. 
+`*splat` matches any path without the root path. If you need to match the root path as well `/`, you can use `/{*splat}`, wrapping the wildcard in braces.
 
 ```js
 // v5
@@ -424,15 +442,15 @@ app.get('/:file{.:ext}', async (req, res) => {
 app.get('/[discussion|page]/:slug', async (req, res) => {
   res.status(200).send('ok')
 })
-``` 
+```
 should be changed to:
 ```js
 app.get(['/discussion/:slug', '/page/:slug'], async (req, res) => {
   res.status(200).send('ok')
 })
 ```
 
-- Some characters have been reserved to avoid confusion during upgrade (`()[]?+!`), use `\` to escape them. 
+- Some characters have been reserved to avoid confusion during upgrade (`()[]?+!`), use `\` to escape them.
 - Parameter names now support valid JavaScript identifiers, or quoted like `:"this"`.
 
 <h3 id="rejected-promises">Rejected promises handled from middleware and handlers</h3>