Add System.Net.Mime.MediaTypeMap (#121776)

Closes #121017

Author: stephentoub

src/libraries/System.Net.Mail/ref/System.Net.Mail.cs

@@ -385,6 +385,13 @@ public static partial class Text
             public const string Xml = "text/xml";
         }
     }
+    public static partial class MediaTypeMap
+    {
+        public static string? GetExtension(System.ReadOnlySpan<char> mediaType) { throw null; }
+        public static string? GetExtension(string mediaType) { throw null; }
+        public static string? GetMediaType(System.ReadOnlySpan<char> pathOrExtension) { throw null; }
+        public static string? GetMediaType(string pathOrExtension) { throw null; }
+    }
     public enum TransferEncoding
     {
         Unknown = -1,

src/libraries/System.Net.Mail/src/System.Net.Mail.csproj

@@ -30,6 +30,7 @@
     <Compile Include="System\Net\Mime\DispositionTypeNames.cs" />
     <Compile Include="System\Net\Mime\HeaderCollection.cs" />
     <Compile Include="System\Net\Mime\MediaTypeNames.cs" />
+    <Compile Include="System\Net\Mime\MediaTypeMap.cs" />
     <Compile Include="System\Net\Mime\MimeBasePart.cs" />
     <Compile Include="System\Net\Mime\SmtpDateTime.cs" />
     <Compile Include="System\Net\Mime\ByteEncoder.cs" />