Maximum attachment size is porly documented

When attaching large files to a page (say, 20+ megabytes), the files get truncated by the default setup in MySQL. That's due to the size of the MEDIUMBLOB column used to store the attachment data. Changing the type to LONGBLOB fixes the problem, but that's not the type of thing people might think about. The instructions for increasing the max_packet_size should mention the BLOB issue.

Ideally, xwiki would return an error if a user tried to upload an attachment that exceeded the storage space.