This is not a new issue, as this is the same functionality we have provided for years on the File: page. But Media Viewer makes it a lot easier for users to accidentally load a huge file. So I think we should seriously consider providing a warning, if it is easy to implement and if we can identify a threshold that is based on data and that is acceptable to our communities.
See [[bugzilla:29967]], “ Better support for viewing and downloading large files” https://bugzilla.wikimedia.org/show_bug.cgi?id=29967