Extract attachments view: Fixed contenttype helper when lookup via filename.

The `content_type_helper` used for displaying the according icon in the `type`
column of the attachments table, has a fallback for attachments with
`application/octet-stream` as contenttype.

But this fallback has not work correctly, because the `globFilename` method
always returns the mimetype itself.

@lukasgraf please have a look ...