[API Proposal]: Add multipart/related to `MediaTypeNames.Multipart`

[CollinAlpert]

Background and motivation

The System.Net.Mime.MediaTypeNames.Multipart type already contains a constant for multipart/form-data which can be used by developers instead of introducing own constants at the risk of typos or duplication.

The same should exist for multipart/related.

API Proposal

namespace System.Net.Mime;

public static class MediaTypeNames
{
    public static class Multipart
    {
         // see https://www.ietf.org/rfc/rfc2387.txt
+        public const string Related = "multipart/related";
         // see https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
+        public const string Mixed = "multipart/mixed"
    }
    public static class Application
    {
         // https://datatracker.ietf.org/doc/html/rfc6713
+        public const string Gzip = "application/gzip";
         // see https://github.com/ndjson/ndjson-spec
+        public const string NDJson = "application/x-ndjson";
    }
    public static class Text
    {
        // see https://github.com/dotnet/runtime/issues/102194
+       public const string EventStream = "text/event-stream";
    }
}

API Usage

using Microsoft.AspNetCore.Http;
using System.Net.Mime;

void M(HttpRequest request)
{
    Console.WriteLine(request.ContentType == MediaTypeNames.Multipart.Related)
}

Alternative Designs

No response

Risks

No response

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Issue Details

---

Background and motivation

The System.Net.Mime.MediaTypeNames.Multipart type already contains a constant for multipart/form-data which can be used by developers instead of introducing own constants at the risk of typos or duplication.

The same should exist for multipart/related.

API Proposal

namespace System.Net.Mime;

public static class MediaTypeNames
{
    public static class Multipart
    {
+        public const string Related = "multipart/related";
    }
}

API Usage

using Microsoft.AspNetCore.Http;
using System.Net.Mime;

void M(HttpRequest request)
{
    Console.WriteLine(request.ContentType == MediaTypeNames.Multipart.Related)
}

Alternative Designs

No response

Risks

No response

Author:	CollinAlpert
Assignees:	-
Labels:	api-suggestion, area-System.Net
Milestone:	-

[ManickaP]

Triage: seems reasonable, but we might consider collecting more together and batch them as we did in #85807.

[CollinAlpert]

Given this would only be introduced in .NET 9 anyway, I have no problem waiting for some more media types.

[CollinAlpert]

Just discovered another one:

namespace System.Net.Mime;

public static class MediaTypeNames
{
    public static class Application
    {
+        public const string Gzip = "application/gzip";
    }
}

[RenderMichael]

What about application/x-ndjson?

[ManickaP]

Keep them incoming. But please also upvote the issue so that we can prioritize accordingly.

[CollinAlpert]

@ManickaP Since the first preview of .NET 9 has been released and there has been no further activity, can we hand this over to API review?

[Bellgrad]

In relation to an issue I submitted today, there is also multipart/mixed missing in system.net.mime.mediatypenames.multipart. Not dramatic but still usefull.