Add MaxUploadLimit to guilds (#2001)

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -372,6 +372,16 @@ namespace Discord.Rest
                 Preconditions.NotNullOrEmpty(attachment.FileName, nameof(attachment.FileName), "File Name must not be empty or null");
             }
 
+            if (channel is ITextChannel guildTextChannel)
+            {
+                var contentSize = (ulong)attachments.Sum(x => x.Stream.Length);
+
+                if (contentSize > guildTextChannel.Guild.MaxUploadLimit)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(attachments), $"Collective file size exceeds the max file size of {guildTextChannel.Guild.MaxUploadLimit} bytes in that guild!");
+                }
+            }
+
             // check that user flag and user Id list are exclusive, same with role flag and role Id list
             if (allowedMentions != null && allowedMentions.AllowedTypes.HasValue)
             {