A colleague of mine has experienced a problem with maximum upload file size being up to 256 KiB in PhpBB
Apparantely the default settings for the PHPBB are that the maximum attachment to be uploaded can be no bigger than 256 KiB
Guess the forum developers decided to set it to such a low value to prevent Denial of Service attacks, anyways if you want to upload a normal file nowadays it will be hardly under a few megabytes.
Thus I still doubt if this decision was really wise, anyways I had to deal with the situation so I found the 256 KiB problem in the PHPBB bug tracking system you can read the thorough thread here
In the mean time to quickly change the default phpbb restrictive behaviour for your PHPBB forum, you should login with administrator.
Then navigate to Admin Control Panel (ACP) and follow the following links to come to the place where you can define the maximum file size for default users in phpbb.
ACP -> General -> Board configuration -> Attachment settings -> Maximum file size
