using var httpClient = new HttpClient();
using var fileContent = new ByteArrayContent(fileBytes);
fileContent.Headers.ContentType = new MediaTypeHeaderValue("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"); //format for exel
var uploadResponse = await httpClient.PostAsync(uploadUrl, fileContent);

this code before calling api.Files.CompleteUploadExternal helped, I don't know why))