Unfortunately the new migration in 2.77.0 appears to have caused all permissions to revert to 000 on uploaded media on at least one site.
In an abundance of caution we have pushed out 2.77.1 which does not carry out that migration. We are working on 2.77.2 which will carry it out properly.
Your files are not lost, you may restore your permissions with chmod 644 public/uploads/attachments/*, if you are using S3 you can do this by restoring public access to each file, for Azure the suffix added to the name to disable each file would need to be removed.
Fortunately this issue was caught quickly so there are probably no instances of the latter two in the wild.