If you are trying to upload big files, but don't succeed check two things:

1. Make sure the Maximum attachtment size is set high enough. You can find that setting under Admin console > Settings > UI (tab)
2. If that setting is correct and your big uploads still fail, please check the apache error log of your virtual host. If it says something like:
   

   Error string not specified yet: Content-Length header (98556532) exceeds configured max_body limit (67108864)

   the the default read_limit variable of libapreq is set too low. (by default this is 64M). To increase it set it to a higher value by adding the following line to your httpd.conf (or modperl.conf if you are using wre):
   

   APREQ2_ReadLimit 1024M

   In this case uploads up to a gigabyte will be accepted by apache.

Please note:

1. The  APREQ_ReadLimit will not override the Maximum attachment size option.
2. The 1024M setting is essentially insane because HTTP is not really designed for transfering such big files. But if you don't want to be hindered by the APREQ_ReadLimit it is an okay value.
3. Setting APREQ_ReadLimit really high potentially opens a DoS attack vector. So if you want to be on the safe side, set it to the maximum file size  you expect to be uploaded.