It seems that the elasticsearch-mapper-attachment plugin has been deprecated in 5.0.0 (Released Oct. 26th, 2016). The documentation recommends using the Ingest Attachment Processor Plugin as a replacement.
To install:
sudo bin/elasticsearch-plugin install ingest-attachment
See How to index a pdf file in Elasticsearch 5.0.0 with ingest-attachment plugin? for information on how to use the Ingest Attachment plugin.