account_archive: unify archive file type

When creating an account archive, there would be no default
suffix. Then, when importing the archives, we would filter
for .gz files by name. The goal is to implement a .jami
file extension for archives across all platforms.

Todo: implement custom MIME type for custom icons.

GitLab: #1256
Change-Id: I202ae5670c928f35b0eaf711abf7249ec3f304b9