document the new mime_type param on file:entries tag

Author: TomJaeger

docs/add-ons/file.md

@@ -128,6 +128,21 @@ You can hard code the file entries tag to show specific files. You may also spec
 
 If you want to display only the files that are in certain folder within an Upload Directory, specify the folder ID using this parameter.
 
+### `mime_type=`
+
+    mime_type="image/jpeg|image/png"
+
+Filter returned files by MIME type. You may specify one or more MIME types; separate multiple values with the pipe character.
+
+Or use "not" to exclude types:
+
+    mime_type="not video/mp4|video/quicktime"
+
+Common examples:
+
+- Images only: `mime_type="image/jpeg|image/png|image/gif"`
+- PDFs only: `mime_type="application/pdf"`
+
 ### `limit=`
 
     limit="30"