The issue is reported against an old version of MantisBT which is no longer supported.
We recommend that you upgrade to the latest stable version [1];

This is not a bug report for MantisBT (you described your current situation, but you did not provide detailed step by step instructions to reproduce the issue).
I am therefore resolving this issue as "no change required".

Feel free to reopen the issue if you are able to reproduce the issue on latest stable version.

The issue might be caused by changing the configuration of the attachment folder, problems on your file system, unintentional deletion of the files, a Mantis bug (e.g. I remember there is/was a problem with attachments when moving issues from one project to another project ), ...

Please use the forums to get support on customizing and using MantisBT (refer to http://www.mantisbt.org/support.php for links and further details).

[1] http://www.mantisbt.org/download.php

Thanks for your quick reply and support.
I find that while making clone of any bug, system is asking to tick Copy bug notes Copy attachments. When i tick 'Copy attachments' then following error occurs
SYSTEM WARNING: copy(258362f2ce0a07fa5699811c218c1c05): failed to open stream: No such file or directory
SYSTEM WARNING: chmod(): No such file or directory

and attachment is missing in clone bug

When i tick 'Copy attachments' then following error occurs

This was a known issue in older versions, but one of the core developers was not able to reproduce it in 2015, see 0007705.