RequestBodyParserMiddleware should respect MAX_FILE_SIZE post value #258

The MAX_FILE_SIZE post value (cast insensitive) can be used to limit the maximum size for each following file upload. This is PHP's default behavior and it probably makes sense to mimic this here.

See also https://stackoverflow.com/questions/1381364/max-file-size-in-php-whats-the-point and https://github.com/php/php-src/blob/1c295d4a9ac78fcc2f77d6695987598bb7abcb83/main/rfc1867.c#L1053

This should not be confused with #257. In particular, this value can easily be spoofed and we have to ensure that this does not allow clients to bypass the server limits.