Enable adding metadata for file attachments

[christian-eriksson]

When attaching a file to a Invoice or Transaction one cannot add a name, description or other meta data to the file. File metadata could be used to provide descriptions or context to the file in reference to the transaction.

Information a user may want to attach as metadata could be:

• references to data in the file that is relevant to the transaction, e.g. what
  lines on a receipt are relevant.
• describe the content of the file, e.g. these are the terms of service provided
  with the customer, version xyz
• describe any connection with external resources that cannot be uploaded, e.g.
  this is a copy of agreement X, original stored in Y.

This would make it easier to understand, review and audit the transactions and the information provided in them.

Without this feature users are left with a few workarounds. For example, keeping a separate register of files with a reference to LedgerSMB. Switching between applications like that is very clunky and time consuming. Another workaround is using the Notes field in LedgerSMB, less clunky but the metadata is not actually connected with the files, and at the moment for GL transactions, you cannot add notes after the transaction is posted but you may add files after posting, which makes notes+files not really synch up in terms of functionality.

[christian-eriksson]

This is a summary of a discussion we previously had in Matrix, I wanted to move it into github but wasn't sure is I should make an issue out of it, a project or a discussion. I figured that one can always make issues from discussions so I put it here :)