diff --git a/restapi.go b/restapi.go
index 6fb1238..59c4907 100644
--- a/restapi.go
+++ b/restapi.go
@@ -1383,9 +1383,10 @@ func (s *Session) ChannelMessageSendComplex(channelID string, data *MessageSend)
 		for i, file := range files {
 			h := make(textproto.MIMEHeader)
 			h.Set("Content-Disposition",
-				`form-data; name="file`+strconv.Itoa(i)+
-					`"; filename="`+quoteEscaper.Replace(file.Name)+
-					`"`)
+				fmt.Sprintf(`form-data; name="file%d"; filename="%s"`,
+					i,
+					quoteEscaper.Replace(file.Name),
+				))
 			contentType := file.ContentType
 			if contentType == "" {
 				contentType = "application/octet-stream"