Documents should have getter for attached files

[laserval]

Currently, getting hold of attachments requires:

1. Getting hold of the RemotePipeline from the current stage
2. Calling getFileNames to find out what attachments, if any, the document has
3. Calling getFile for reach file you want
4. Getting the inputstream from the returned DocumentFile
5. Streaming the file to wherever it is needed

[laserval]

There is a problem here, in that documents don't contain the attachment. To fetch an attachment, a request must be made to Core to get the content.

A utility method could be added to RemotePipeline that simplifies this to a single step getFiles that returns all attachments as DocumentFiles.

To get an actual attachment-getter for LocalDocument, something more needs to change.