[imp] Can't Attach Files Larger Than 1MB When Composing Message

[End User]

Hello Everyone,

I have recently installed Horde v2.1 and Imp v3.1. My system specs follow:

OS: Redhat Linux 8.0
Platform: x86
PHP Ver: 4.2.2
Apache Ver: 2.40
Horde Ver: 2.1
Imp Ver: 3.1

I have edited the following directives in my php.ini file and restarted
apache after each edit:

post_max_size = 50M
file_uploads = On
max_execution_time = 60
memory_limit = 256M (I have tried more reasonable values as well ie: 64MB)
upload_tmp_dir =/tmp
upload_max_filesize = 50M (I have even tried to increase/decrease this to a
larger/smaller value)

Everything seems to be working fine except when I try to attach a file
larger than 1MB (actually around 500kb before encoding). When I try to
attach a large file I get redirected to a "The page cannot be displayed"
page in my browser (gg IE). I have extensively searched the web and,
although there are numerous posts on this topic, I have been unsuccessful in
finding one that matches mine.

Does anyone have an idea about what the problem might be or, at least, which
component is at fault? I am ruling out apache because small attachments work
fine. Is there something in one of the horde or imp configuration files that
would limit the allowed attachment size? If so, why would I not get the
error message, "file too large ....", defined in the compose.php file?

Thanks in advance