Attachment behavior fix for Firefox

server/message_cache.go

server/message_cache_test.go

server/server.go

server/server_test.go

server/visitor.go

util/limit.go

util/peek.go

util/peek_test.go

web/src/app/Api.js

web/src/app/utils.js

web/src/components/App.js

web/src/components/SendDialog.js