Auto-hide the attachments upload status

During an usability testing session one user complained that she needed to do an extra step in order to hide the upload status. 

Her argument was that when deleting attachments, we receive a message, but we don't have an extra action to perform. 

She wanted to have the upload status be visible, but if things go well, it should auto-hidden. 

She mentioned that this would make the wiki feel more rapid and consistent with the delete behaviour. 

Note: if the upload has errors, we should keep the current behaviour and display what attachments were successful and which had problems.