I assume that your application is not using default upload input tag. So, you can not use sendKeys to fill in the file.
If your application uses default upload input tag, that looks something like below,
you can use driver.sendKeys("Your/path/to/file");
It will take care of sending a file to the server and upload the file to the application.
But, if your application uses customized file upload functionality, implementing this is really difficult. As, browserstack or any third party as far as I know do not allow any file to be uploaded to their server.
Hope that answers your question.