Provides the data for an attachment. Please note that there will
be a strong reference retained for the provider for each Attachment
it provides, so do try to keep the object light. Also, reuse objects
if possible.
The Provider needs to be thread-safe.
author: Janne Jalkanen since: 2.5.34
Returns a stream of data for this attachment. The stream will be
closed by AttachmentServlet.
Parameters: context - A Wiki Context Parameters: att - The Attachment for which the data should be received. InputStream for the data. throws: ProviderException - If something goes wrong internally throws: IOException - If something goes wrong when reading the data