So in webmail, when you upload an image / file to attach it to an email, you expect it to be renamed? I don't.

I email images as attachments very, very frequently. I go through the browser's file picker and I pick out the photo by its filename. I would be surprised and angry if somewhere along the way the filename got changed to some random string without my knowledge and consent.

In fact, I often refer to the name of the photo in the body of the email (e.g., "front_before.jpg shows the front of the car when I picked it up, front_after.jpg shows it after the accident.")

I imagine this is an extremely common use case.