How to store IPFS hash using bytes32?

Your example shows storing an IPFS identity using it's alphanumeric encoding (Qm...), which is the same Base58 encoding that Bitcoin uses. However, what it's representing at its core is a number (the hash). Storing the identifier in the Base58 format needs to be a String because it includes letters (and what actually gets saved is the ASCII code for each alphanumeric character in the identifier).