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 SMF 2.0:

[SMF 2.0 upgrade script](https://github.com/SimpleMachines/SMF2.1/blob/release-2.1/other/upgrade_2-0_mysql.sql)

Signed-off-by: Noteworthy Software, Inc <online@noteworthysoftware.com>
Merge pull request #42 from nwsw/fix

Fix attachment mime_type and fileext SMF 1.1->ElkArte