Use chunked upload for large files

[s7eph4n]

If a file exeeds post_max_size or upload_max_filesize set in php.ini the upload will fail without proper error message.

It seems that PHP does not raise an error when the file is too large and ApiUpload is not explicitly testing this error condition, so no error message is generated.
MediaWiki's own $wgMaxUploadSize is checked however.

Workaround for the moment: Raise the php.ini settings above the largest expected file size.