Attachment upload failure gives no feedback to user

When an attachment upload fails (disk full/max_allowed_packet to small/etc..), no feedback is given to the user of this failure (logs of the app server show the exception), as this is running in AJAX mode. Only the attachment list is reloaded which does not show the upload.

There should be a check of the result of that upload and a message should be shown to the user that it failed. Additionally the reason of the failure should be explained (particularly max_allowed_packet)

This is critical as it could lead users to think their document was uploaded when it isn't.