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