This views has multiple custom features such as file upload, attachment
viewer and download.
A lot of files have been rendered obsolete and have been deleted,
notably `file_upload_mixin` which has been replaced by the
file_upload service and hooks.
closesodoo/odoo#99769
Related: odoo/enterprise#31145
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>