Have you tried using files as attachment storage (instead of database, as is the default, see config_defaults.php)? I don't know about any memory limit that you can change, but if there is any, it should be in php.ini.

YA,I have changed every necessary varibles including php.ini .
But,it still can't work.

there's a memory limit in php.ini (30MB in your case):
http://www.php.net/manual/en/configuration.directives.php#ini.memory-limit
and you need to set it to fit the largest upload you want plus some safeguard for the rest of the script.

I ran into this yesterday, and solved it by changing a few values:

In the default Red Hat apache I was using for this client, I noticed the following structure in their httpd.conf:

<Files *.php>
SetOutputFilter PHP
SetInputFilter PHP

LimitRequestBody 524288

</Files>

I commented that last line out. Originally, I was getting a 'Document contains no data' message. Once I fixed that, I got further. But not quite...

In php.ini, there is also a few things that need to be adjusted:

If your file is LARGE, make sure to check 'max_execution_time', 'max_input_time', and 'memory_limit'. Increase them as necessary. Also make sure 'file_uploads' is set to "On".

That then allowed me to get the file into Mantis, but then it crashed with the following error:

'Database query failed. Error received from database was 0002006: MySQL server has gone away for the query: INSERT INTO mantis_bug_file_table'

This was solved by putting the following in my /etc/my.cnf (MySQL global confiuration file)

max_allowed_packet = 8M

Then I could upload the file into the system. So the order is:

Make sure Apache allows a POST size that permits your file to be accepted (LimitRequestBody).

Make sure PHP has enough memory to handle parsing the file (memory_limit).

And make sure MySQL (if using that) has a large enough "bite" to accept the blob (max_allowed_packet)

Hopefully this helps.

I just hit a similar issue in Mantis 0.19.0

When I tried to upload a 2.2MB file I got a similar error. I had to boost the "memory_limit" value in php.ini from 8MB all the way to 16MB to fix the problem. That is a large boost for a 2.2MB file, and it made me think that mantis is not allocating memory efficiently when using the DATABASE upload storage method.

I switched to FILE uploads in config_defaults.php and was then able to change the php "memory_limit" back to 8M. Maybe its the Mysql driver code and not Mantis, but it's worth looking into (I'll do it if I get time).