Fix attachment mime_type and fileext SMF 1.1->ElkArte

SMF 1.1 forum attachments do not have mime_type or fileext settings, so they must be calculated. This updates the moveAttachment function to setup these fields in a manner somewhat consistent with what is done by the [SMF 2.0 upgrade script](https://github.com/SimpleMachines/SMF2.1/blob/release-2.1/other/upgrade_2-0_mysql.sql).

Without this change, all existing image attachments lack a mime_type after importing/converting. This causes big problems for Internet Explorer.

Signed-off-by: Noteworthy Software, Inc <online@noteworthysoftware.com>