Set Stream.Position in FileAttachment-ctor to 0 (#2063)

Co-authored-by: Christoph Liebender <c.liebender@vision4quality.de>

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;
         }