Modifying the file stream mid process

Hello,

I am using multer to allow uploading of images from my webapp to our storage. I want to add a watermark to the images before they are uploaded.

I have been able to take the stream from the file object multer exposes and to apply the watermark to the file buffer inside of the stream, however I cannot connect this modified stream back to the file object from multer.

Is there any guidance on how this could be done?