If I upload file attachment files/URLs with the same filename, then an upload will overwrite a previous upload with the same name and only the last uploaded file remain in the post. Files on sites like eBay use the "pathname" to distinguish different files that have the same URL "filename". For example, these 4 files cannot coexist as file attachments in a post.
I suppose uploaded files from a post are stored in the same folder on the server with the uploaded name (or in the database with the filename as key) so there cannot be more than 1 file with the same name. I'm using Classic Mode/Full Site if that matters.
I can live with this issue. There are workarounds like downloading everything locally, giving them different names, then uploading. I just want to let others know about this so they don't waste time attaching a dozen images from like eBay