Replace bad file type error with constant

bot.go

@@ -146,7 +146,7 @@ func (bot *BotAPI) UploadFile(endpoint string, params map[string]string, fieldna
 
 		ms.WriteReader(fieldname, f.Name, int64(len(data)), buf)
 	default:
-		return APIResponse{}, errors.New("bad file type")
+		return APIResponse{}, errors.New(ErrBadFileType)
 	}
 
 	method := fmt.Sprintf(APIEndpoint, bot.Token, endpoint)

configs.go

@@ -39,6 +39,12 @@ const (
 	ModeMarkdown = "Markdown"
 )
 
+// Library errors
+const (
+	// ErrBadFileType happens when you pass an unknown type
+	ErrBadFileType = "bad file type"
+)
+
 // Chattable is any config type that can be sent.
 type Chattable interface {
 	values() (url.Values, error)