move extension method to be under core project, make spoiler prefix a const

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -60,10 +60,5 @@ namespace Discord.Commands
             }
             return false;
         }
-        /// <summary>
-        ///     Gets wheter the message's attachments are spoilers or not.
-        /// </summary>
-        public static bool IsSpoiler(this IAttachment attachment)
-            => attachment.Filename.StartsWith("SPOILER_");
     }
 }

src/Discord.Net.Core/Entities/Messages/IAttachment.cs

@@ -5,6 +5,11 @@ namespace Discord
     /// </summary>
     public interface IAttachment
     {
+        /// <summary>
+        ///     The prefix applied to files to indicate that it is a spoiler.
+        /// </summary>
+        public const string SpoilerPrefix = "SPOILER_";
+
         /// <summary>
         ///     Gets the ID of this attachment.
         /// </summary>

src/Discord.Net.Core/Extensions/AttachmentExtensions.cs

@@ -0,0 +1,11 @@
+namespace Discord
+{
+    public static class AttachmentExtensions
+    {
+        /// <summary>
+        ///     Gets wheter the message's attachments are spoilers or not.
+        /// </summary>
+        public static bool IsSpoiler(this IAttachment attachment)
+            => attachment.Filename.StartsWith(IAttachment.SpoilerPrefix);
+    }
+}

src/Discord.Net.Rest/API/Rest/UploadFileParams.cs

@@ -30,8 +30,8 @@ namespace Discord.API.Rest
         {
             var d = new Dictionary<string, object>();
             var filename = Filename.GetValueOrDefault("unknown.dat");
-            if (IsSpoiler && !filename.StartsWith("SPOILER_"))
-                filename = filename.Insert(0, "SPOILER_");
+            if (IsSpoiler && !filename.StartsWith(IAttachment.SpoilerPrefix))
+                filename = filename.Insert(0, IAttachment.SpoilerPrefix);
             d["file"] = new MultipartFile(File, filename);
 
             var payload = new Dictionary<string, object>();