7z not supported

Dear Microsoft,

please remove the whole file extensions whitelist. It is useless and doesn't increase the security.

A lot of important and modern file types are missing (e.g. `.7z`).

Start by locating the file-extension whitelist in the file-attachment-element code and trace how it is applied to drag-and-drop and file-input attachments. Reproduce the current rejection of a .7z file, then verify the requested whitelist behavior and its existing tests; done means the intended modern file types are accepted without breaking attachment handling.

Written by the indexing model from the issue text.