We recommend using the binary cache for this, since it gives you a fast and easy way to get large pieces of data in and out of MultiChain without converting to hexadecimal and using the JSON-RPC API.
See section 6 of Getting Started for an example: https://www.multichain.com/getting-started/
For writing, use createbinarycache, then put the data into the cache file directly, then publish using the {"cache":"Ev1HQV1aUCY"} parameter form, then deletebinarycache.
For reading, use createbinarycache, txouttobinarycache, then read the data from the cache file directly, then deletebinarycache.
If you're working with large pieces of data we recommend publishing the data off-chain, by passing the offchain in the options parameter when publishing.