[message] fixed 'attachment unshown' bug, and only allow files as attachment
Squashed commit of the following:

commit 1e791a50186ae10df65cf2f72b723f85301bf36a
Author: swpd <swpdtz@gmail.com>
Date:   Mon Dec 23 21:29:42 2013 +0800

    [group] allow staff to choose an existing group library to be used as group wiki

commit 23a3bc6ee4e95997ea402076d01980661a6aa2f4
Author: swpd <swpdtz@gmail.com>
Date:   Mon Dec 23 13:59:58 2013 +0800

    [wiki] allow user to choose an existing personal library to be used as personal wiki

commit fa3df2cf76b79a0593bb1eda2b413774c63b2676
Author: swpd <swpdtz@gmail.com>
Date:   Sun Dec 22 02:39:20 2013 +0800

    [wiki] generate within-page anchors for h1 and h2 tags automatically

commit 034a75626f825186a48b398e231f73185a54635c
Author: swpd <swpdtz@gmail.com>
Date:   Sun Dec 15 16:40:42 2013 +0800

    [group] allow group user to create wiki hierarchy index

commit 590b5c1488573d83eab636b0747d4f64d5f9d589
Author: swpd <swpdtz@gmail.com>
Date:   Sun Dec 15 16:23:46 2013 +0800

    [wiki] allow user to create personal wiki hierarchy index
[wiki] improvement & bug fix for header links, index, & creating wiki by choosing a lib
[wiki index] improved ui
[wiki improve] index: show tip & create btn when no index; improved img ui
Revert "WIP: multi-tenancy"

This reverts commit 37d57c16391d769b8844d964c8533588267a4591.
Merge branch '2.1'