How it is possible to limit total size of attachments. For example, total size 5Mb, max file size 5Mb, but we can load 2 or more files with length less 5Mb

ive set the max size to 10MB but it does not allow files even only up to 9.6MB to be uploaded. i've set a temporary value on the max size so as to allow uploading of files up to 10MB. is this a bug on control or do i need to set a certain value on the maxsizekb property to allow uploading up to 10MB?