Deletes a document from a draft contract

Removes one attachment from a draft contract. The stored file is deleted unless a contract, another draft contract or a milestone still uses it, or the file belongs to another part of Rillet. Works for files uploaded through the API or the Rillet application, or imported from Salesforce. A file imported from Salesforce stays removed when the draft contract is imported again, unless a newer version of it is uploaded in Salesforce.