Fix 0000838: Show attached images as images instead of links.

M config_defaults_inc.php

• New config option (preview_attachments_inline_max_size): Configure the maximum size for an attachment to be
  viewed inline. This feature can be disabled by setting this value to 0. Default is 0.

M core/file_api.php

• (file_list_attachments): Added support for attachment previewing.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2023 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9