[imp] Session expired when attaching a file

[SSRI]

>>
>> Why end user doesn't get a warning message ? Usually, web  
>> applications warns you that the file is too big.
>
> This is a PHP limitation.  You will need to bug them for the explanation.
>

We were thinking about implementing an "uploading too large of files  
warning" ( that exists for example in Mediawiki :  
http://www.mediawiki.org/wiki/Manual:$wgUploadSizeWarning )