The content-type is important for Riak KV: Various clients will use it to identify the difference between a response where the whole body is the key, or a response that contains siblings

It will also allow you to version your objects better. Here are a few suggestions (of course replace the text between the < >):

• application/octet-stream – this is usually used for binary data, and is the easiest thing to set the content-type to.
• application/vnd.<company name>.<useful type name here> – a vendor-specific type, which you can create yourself
• application/vnd.<company name>.<useful type name here>.v<version number> – another vendor-specific type, which supports versioning.

Source: by Sam Elliott in the mailing list