Use identifier for attachments export

lib/export/export_collections.rb

@@ -61,7 +61,7 @@ module Export
 
           # write all attachemnts into an attachments directory
           @attachments.each do |attachment|
-            attachment_path = File.join('attachments', attachment.filename)
+            attachment_path = File.join('attachments', attachment.identifier)
             zip.put_next_entry attachment_path
             zip.write attachment.read_file
             description += "#{attachment.checksum} #{attachment_path}\n"

lib/import/import_collections.rb

@@ -360,7 +360,7 @@ module Import
         attachable = @instances.fetch(attachable_type).fetch(attachable_uuid)
 
         # construct file path
-        file_path = File.join(@directory, 'attachments', fields.fetch('filename'))
+        file_path = File.join(@directory, 'attachments', fields.fetch('identifier'))
 
         # create the attachment
         attachment = attachable.attachments.create!(