Cannot specify Content-Type for charts tarball.

[bashims]

Chart tarballs are uploaded using the "binary/octet-stream" content-type. It would be nice to be able to specify a content-type like "application/x-tar" or "application/x-gzip"

Example HeadObject() response

{                                                                                                                     
    "AcceptRanges": "bytes",                                                                                          
    "ContentType": "binary/octet-stream",                                                                             
    "LastModified": "Mon, 15 Oct 2018 18:51:03 GMT",                                                                  
    "ContentLength": 25232,                                                                                           
    "VersionId": "null",                                                                                              
    "ETag": "\"11f69dd7d7dd80f72e5a6b5c6f7119bf\"",                                                                   
    "Metadata": {                                                                                                     
        "chart-metadata": "..................",          
        "chart-digest": "............................"                            
    }                                                                                                                 
}