feat: add support for attachment descriptions

Myriad/Rest/Types/MultipartFile.cs

@@ -2,5 +2,5 @@ using System.IO;
 
 namespace Myriad.Rest.Types
 {
-    public record MultipartFile(string Filename, Stream Data);
+    public record MultipartFile(string Filename, Stream Data, string? Description);
 }

Myriad/Rest/Types/Requests/ExecuteWebhookRequest.cs

@@ -8,6 +8,7 @@ namespace Myriad.Rest.Types.Requests
         public string? Username { get; init; }
         public string? AvatarUrl { get; init; }
         public Embed[] Embeds { get; init; }
+        public Message.Attachment[] Attachments { get; set; }
         public AllowedMentions? AllowedMentions { get; init; }
     }
 }