[BUGFIX] lower POST size when ̀PHP's upload_max_filesize is too low.

When post_max_size == upload_max_filesize, Jifeau sends too large request (files + headers) and serveur still returns 200.
Jirafeau does not see any files and return Error 23.
Javascript client now detect this error and retry with a lower file size to make room for headers.

closes #240

Signed-off-by: Jerome Jutteau <jerome@jutteau.fr>