Add endpoint for retrieving "folder" media types. (#16345)

* Add endpoint for retrieving "folder" media types.

* The system Folder media type should always be considered a "folder"

tests/Umbraco.Tests.Integration/Umbraco.Core/Services/MediaTypeEditingServiceTests.GetFolderMediaTypes.cs

@@ -0,0 +1,84 @@
+using NUnit.Framework;
+using Umbraco.Cms.Core;
+using Umbraco.Cms.Core.Models;
+
+namespace Umbraco.Cms.Tests.Integration.Umbraco.Core.Services;
+
+public partial class MediaTypeEditingServiceTests
+{
+    [Test]
+    public async Task Can_Get_Default_Folder_Media_Type()
+    {
+        var folderMediaTypes = await MediaTypeEditingService.GetFolderMediaTypes( 0, 100);
+        Assert.AreEqual(1, folderMediaTypes.Total);
+        Assert.AreEqual(Constants.Conventions.MediaTypes.Folder, folderMediaTypes.Items.First().Alias);
+    }
+
+    [Test]
+    public async Task Can_Yield_Multiple_Folder_Media_Types()
+    {
+        var imageMediaType = MediaTypeService.Get(Constants.Conventions.MediaTypes.Image);
+
+        var createModel = MediaTypeCreateModel("Test Media Type", "testMediaType");
+        createModel.Description = "This is the Test description";
+        createModel.Icon = "icon icon-something";
+        createModel.AllowedAsRoot = true;
+        createModel.Properties = [];
+        createModel.AllowedContentTypes = new[]
+        {
+            new ContentTypeSort { Alias = imageMediaType.Alias, Key = imageMediaType.Key }
+        };
+
+        await MediaTypeEditingService.CreateAsync(createModel, Constants.Security.SuperUserKey);
+
+        var folderMediaTypes = await MediaTypeEditingService.GetFolderMediaTypes( 0, 100);
+        Assert.AreEqual(2, folderMediaTypes.Total);
+        Assert.Multiple(() =>
+        {
+            var aliases = folderMediaTypes.Items.Select(i => i.Alias).ToArray();
+            Assert.IsTrue(aliases.Contains(Constants.Conventions.MediaTypes.Folder));
+            Assert.IsTrue(aliases.Contains("testMediaType"));
+        });
+    }
+
+    [Test]
+    public async Task System_Folder_Media_Type_Is_Always_Included()
+    {
+        // update the system "Folder" media type so it does not pass the conventions for a "folder" media type
+        // - remove all allowed child content types
+        // - add an "umbracoFile" property
+        var systemFolderMediaType = MediaTypeService.Get(Constants.Conventions.MediaTypes.Folder)!;
+        var updateModel = MediaTypeUpdateModel(Constants.Conventions.MediaTypes.Folder, Constants.Conventions.MediaTypes.Folder);
+        updateModel.Properties = new[]
+        {
+            MediaTypePropertyTypeModel("Test Property", Constants.Conventions.Media.File)
+        };
+        updateModel.AllowedContentTypes = [];
+
+        var updateResult = await MediaTypeEditingService.UpdateAsync(systemFolderMediaType, updateModel, Constants.Security.SuperUserKey);
+        Assert.IsTrue(updateResult.Success);
+
+        // despite the system "Folder" media type no longer living up to the "folder" media type requirements,
+        // it should still be considered a "folder"
+        var folderMediaTypes = await MediaTypeEditingService.GetFolderMediaTypes( 0, 100);
+        Assert.AreEqual(1, folderMediaTypes.Total);
+        Assert.AreEqual(Constants.Conventions.MediaTypes.Folder, folderMediaTypes.Items.First().Alias);
+    }
+
+    [Test]
+    public async Task Folder_Media_Types_Must_Have_Allowed_Content_Types()
+    {
+        var createModel = MediaTypeCreateModel("Test Media Type", "testMediaType");
+        createModel.Description = "This is the Test description";
+        createModel.Icon = "icon icon-something";
+        createModel.AllowedAsRoot = true;
+        createModel.Properties = [];
+        createModel.AllowedContentTypes = [];
+
+        await MediaTypeEditingService.CreateAsync(createModel, Constants.Security.SuperUserKey);
+
+        var folderMediaTypes = await MediaTypeEditingService.GetFolderMediaTypes( 0, 100);
+        Assert.AreEqual(1, folderMediaTypes.Total);
+        Assert.AreEqual(Constants.Conventions.MediaTypes.Folder, folderMediaTypes.Items.First().Alias);
+    }
+}