public static class Attachment.DefaultImpls
An attachment is a ZIP (or an optionally signed JAR) that contains one or more files. Attachments are meant to contain public static data which can be referenced from transactions and utilised from contracts. Good examples of how attachments are meant to be used include:
Calendar data
Fixes (e.g. LIBOR)
Smart contract code
Legal documents
Facts generated by oracles which might be reused a lot.
At the moment, non-ZIP attachments are not supported. Support may come in a future release. Using ZIP files for attachments makes it easy to ensure data on the ledger is compressed, which is useful considering that attachments may be widely replicated around the network. It also allows the jarsigner tool to be used to sign an attachment using ordinary certificates of the kind that many organisations already have, and improves the efficiency of attachment resolution in cases where the attachment is logically made up of many small files - e.g. is bytecode. Finally, using ZIPs ensures files have a timestamp associated with them, and enables informational attachments to be password protected (although in current releases password protected ZIPs are likely to fail to work).