Ensure UploadFile is always a seekable stream.

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -494,13 +494,8 @@ namespace Discord.API
 
             if (args.Content.GetValueOrDefault(null) == null)
                 args.Content = "";
-            else if (args.Content.IsSpecified)
-            {
-                if (args.Content.Value == null)
-                    args.Content = "";
-                if (args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
-                    throw new ArgumentOutOfRangeException($"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", nameof(args.Content));
-            }
+            else if (args.Content.IsSpecified && args.Content.Value?.Length > DiscordConfig.MaxMessageSize)
+                throw new ArgumentOutOfRangeException($"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", nameof(args.Content));
 
             var ids = new BucketIds(channelId: channelId);
             return await SendMultipartAsync<Message>("POST", () => $"channels/{channelId}/messages", args.ToDictionary(), ids, clientBucket: ClientBucketType.SendEdit, options: options).ConfigureAwait(false);

src/Discord.Net.Rest/Net/DefaultRestClient.cs

@@ -101,7 +101,14 @@ namespace Discord.Net.Rest
                         if (p.Value is MultipartFile)
                         {
                             var fileValue = (MultipartFile)p.Value;
-                            content.Add(new StreamContent(fileValue.Stream), p.Key, fileValue.Filename);
+                            var stream = fileValue.Stream;
+                            if (!stream.CanSeek)
+                            {
+                                var memoryStream = new MemoryStream();
+                                await stream.CopyToAsync(memoryStream).ConfigureAwait(false);
+                                stream = memoryStream;
+                            }
+                            content.Add(new StreamContent(stream), p.Key, fileValue.Filename);
                             continue;
                         }