Minor UI adjustment for attachments
Added ability to upload attachment
Deleting the attachment now also removes the downloaded data
When retrieving attachments, delete old attachments that doesn't exists
If the user deletes the attachment using the web client, the app will
now be able to detect that change and delete the old attachment entries
from the database and the file from the local storage if it's been
downloaded
Empty attachment list is represented as empty ArrayList instead of null
This should fix the crash from previous commit when retrieving an item
doesn't have any attachments from OnTime server