onDownload throws error 'Cannot read property 'download' of undefined', when passing custom body in buildBody function

## Expected Behavior

## Current Behavior

## Steps to Reproduce (for bugs)

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## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | |
| MUI-datatables | |
| React | |
| browser | |
| etc | |

Start by locating the buildBody function and the onDownload entry point in the JavaScript source, then reproduce the error while passing a custom body. Done means custom-body downloads no longer throw the reported undefined-property error; add or update a regression test if the repository has coverage for this behavior.

Written by the indexing model from the issue text.