diff --git a/file.go b/file.go
index dc7ffbb..14c40f9 100644
--- a/file.go
+++ b/file.go
@@ -9,7 +9,7 @@ import (
 type File struct {
 	FileID   string `json:"file_id"`
 	UniqueID string `json:"file_unique_id"`
-	FileSize int    `json:"file_size"`
+	FileSize int64  `json:"file_size"`
 
 	// FilePath is used for files on Telegram server.
 	FilePath string `json:"file_path"`
diff --git a/sendable.go b/sendable.go
index a91233e..38d8b48 100644
--- a/sendable.go
+++ b/sendable.go
@@ -87,7 +87,7 @@ func (d *Document) Send(b *Bot, to Recipient, opt *SendOptions) (*Message, error
 	b.embedSendOptions(params, opt)
 
 	if d.FileSize != 0 {
-		params["file_size"] = strconv.Itoa(d.FileSize)
+		params["file_size"] = strconv.FormatInt(d.FileSize, 10)
 	}
 	if d.DisableTypeDetection {
 		params["disable_content_type_detection"] = "true"