Why is a generated file (via SourceGenerator) in .NET7-Windows csproj file not included, if a local namespace in xaml file is used? - wpf

Question

The .NET7-Windows project with SourceGenerator e.g. MimeTypes is working.
When adding a local namespace to the WPF-UserControl the generated file is not included anymore.
How can I modify the project file, that all generated files are automatically included?
Reproducable Example:
Example.csproj:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net7.0-windows</TargetFramework>
<LangVersion>10</LangVersion>
<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
<UseWindowsForms>true</UseWindowsForms>
<UseWPF>true</UseWPF>
<NoWarn>CA1416</NoWarn>
</PropertyGroup>
<PropertyGroup>
<OutputType>Library</OutputType>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="MimeTypes" Version="2.4.0">
<PrivateAssets>all</PrivateAssets>
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>
</ItemGroup>
</Project>
UserControl1.xaml:
<UserControl x:Class="Example.UserControl1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
<Grid/>
</UserControl>
UserControl1.xaml.cs:
using System.Windows.Controls;
namespace Example
{
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
var x = MimeTypes.GetMimeType("Testfile.exe");
}
}
}
This is working as expected, but when adding:
xmlns:local="clr-namespace:Example"
The generated MimeTypes file is not included anymore, which leads to the compiler error CS0103 (The name 'MimeTypes' does not exist in the current context)