there is a setting you can make in config_inc.php such as :
$g_max_file_size = 9000000; # 9 MB

Be aware that the limit set in PHP.ini has preference over the one in mantis, check this seting for php:
upload_max_filesize = 128M

Where is the php.ini file in windows computer?

Thx

most likely in the php directory, sometimes in de windows directory

And this configuration also serves to upload files to the project?

martinfdan, as this is not a bug or feature request for MantisBT (you are asking for help on how to configure your runtime environment) I am resolving this issue.
Please use the forums, the help mailing list or IRC to get support on customising and using MantisBT.
http://www.mantisbt.org/support.php
Have a look at various issues related to this question, for example 0008867 where paramter max_allowed_packet is mentioned which you have to set if you use mySQL to store your attachments.