Project exports fail when uploads have been migrated to object storage

Reported in https://gitlab.com/gitlab-com/support-forum/issues/3646.

You can see this file was exported to object storage, but the project export failed when it tried to access that file:

*** PRODUCTION *** production> proj.uploads.where("path like '%logo%'")                
=> #<ActiveRecord::AssociationRelation [#<Upload id: 4135477, size: 1895, path: "project/avatar/179611/gnutls-logo-nobackground.png", checksum: nil, model_id: 179611, model_type: "Project", uploader: "AvatarUploader", created_at: "2018-06-26 20:27:22", store: 2, mount_point: "avatar", secret: nil>]>

/cc: @jprovaznik, @mbergeron, @smcgivern