set stream.position in FileAttachment ctor to 0

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

@@ -32,6 +32,11 @@ namespace Discord
             FileName = fileName;
             Description = description;
             Stream = stream;
+            try
+            {
+                Stream.Position = 0;
+            }
+            catch { }
             IsSpoiler = isSpoiler;
         }