Supporting updates to files

[slifty]

Right now Permanent does not have a mechanism for updating files associated with an Archive Record.

This means that the sftp service responds to any attempt to open an existing file for "write" with a no-permission error.

So long as Permanent doesn't make this possible, that implementation is correct. That said I at least wanted to open an issue to track the use case, as we know there will be situations where people would like to edit an existing file.

[kfogel]

This is related to https://permanent.atlassian.net/browse/PER-9171.