You must change the value in the Global Parameters screen.
But this is not enough, you also have to change configuration in php.ini to increase upload_max_filesize and post_max_size

Thanks for your help

it works perfectly

Although I did all the settings from parameter ($ param Max Attachment Size)
and the two settings in php.ini (upload_max_filesize and post_max_size) so that the attached file size matches the size set by these parameters but still I have a file size of the problem exceeds this limit

You must also increase parameter in Global Parameter screen.

Hello
yes I changed the "param Max Attachment Size" Global Parameter screen and (upload_max_filesize and post_max_size) in php.in 15 GO

But still I can not attach files whose size + 6MB
How to solve this problem

Change values of

upload_max_filesize 

and

post_max_size

in php.ini

yes i change o value of upload_max_filesize
and
post_max_size but no result

And did you restart you PHP after the change ?