Attemping to upload a chart to Helm, I got back the message "Unsupported assetKind: HELM_INDEX". I thought maybe validation was blocking somehow so I tried turning that off but no dice. This seems to be the message generated if you add a file type not associated with Helm (e.g. I saw the same behavior trying to upload a jar).
I do not think this is a very user friendly message however, it really gave me no clue this was the problem, in fact, I suspected my chart was corrupted/malformed somehow (which I guess it is, but I think the message could be more clear).
Filing for analysis.