Trouble with attatchment ID.

My site require numeric attachment id of file. I always used

Media Library>File(show)>Attachment post url

to know the file’s attachment id.. But I don’t know why that url is rewritten in https://example.com/custom_post/post-name/attachment/name-of-attatchment
In past it was something like https://example.com/attatchment.php?id=xx (I don’t remember the default shape of url, but it was something like that.)
So how to know attachment id of file? and why this happened?
* My url structure is custom from last 10 days as my site’s age. I didn’t touch that from then and the problem is occurring today.