signingUrlHeaders has the type
signingUrlHeaders?: {
additional: object;
} | undefined
But it appears that what it expects is more of a Record<string, unknown> which is incompatible with the aforementioned type. I was able to get my header to work by doing the following:
const headers = { Authorization: `Bearer ${accessToken}` }
<ReactS3Uploader
// ...
signingUrlHeaders={headers as any}
// ...
/>