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found it: Upload lib can be configured by objectKey: '.k'
details:
*data: {key: file, otherInfo: uploadInfo},
/*
This is to accommodate server implementations expecting nested data object keys in .key or [key] format.
Example: data: {rec: {name: 'N', pic: file}} sent as: rec[name] -> N, rec[pic] -> file
data: {rec: {name: 'N', pic: file}, objectKey: '.k'} sent as: rec.name -> N, rec.pic -> file */
objectKey: '[k]' or '.k' // default is '[k]'
Source:stackexchange.com