I think should have custom field in Header in namespace ChunkUpload

[hiro2k-dev]

in \index.d.ts

export interface Header {
        "x-chunk-number": number;
        "x-chunk-total-number": number;
        "x-chunk-size": number;
        "x-file-name": string;
        "x-file-size": number;
        "x-file-identity": string;
        "custom": Object
    }

in \src\index.js

constructor(props) {
        this.data = {
            path: String(props.path),
            size: parseInt(props.size),
            fileName: String(props.fileName),
            fileSize: parseInt(props.fileSize),
            fileIdentity: this.generateFileIdentity(),
            fileShortId: null,
            destinationPath: RNFS.TemporaryDirectoryPath,
            totalNumber: 0,
            custom: props.custom,
        };

getHeaders(index) {
        return {
            "x-chunk-number" : index,
            "x-chunk-total-number" : this.data.totalNumber,
            "x-chunk-size" : this.data.size,
            "x-file-name" : this.data.fileName,
            "x-file-size" : this.data.fileSize,
            "x-file-identity" : this.data.fileIdentity,
            "custom": this.data.custom
        }
    }