Introducing memory file upload handling

This change introduces memory file upload handling and works nearly similar to django's file upload handler logic.

The memory file handler is the default file handler which holds the file in memory. When the maximum size (simple_bridge_max_file_in_memory_size) has reached the memory file handler switches to the temporary file handler that works as before for uploaded files.

It's possible to define custom file handlers. Setting the custom file handlers over configuration like django would be a nice addition. Conributions are welcome.