Here is my code to convert bot file id to media object to download big files using GramJs. You can integrate it.

import bigInt from "big-integer";
import { Api } from "telegram";
import { Buffer } from "buffer";

const WEB_LOCATION_FLAG = 1 << 24; // substitute with actual value
const FILE_REFERENCE_FLAG = 1 << 25; // substitute with actual value

enum FileType {
    THUMBNAIL = 0,
    CHAT_PHOTO = 1, // ProfilePhoto
    PHOTO = 2,
    VOICE = 3, // VoiceNote
    VIDEO = 4,
    DOCUMENT = 5,
    ENCRYPTED = 6,
    TEMP = 7,
    STICKER = 8,
    AUDIO = 9,
    ANIMATION = 10,
    ENCRYPTED_THUMBNAIL = 11,
    WALLPAPER = 12,
    VIDEO_NOTE = 13,
    SECURE_RAW = 14,
    SECURE = 15,
    BACKGROUND = 16,
    DOCUMENT_AS_FILE = 17,
}

// Photo-like file ids are longer and contain extra info, the rest are all documents
const PHOTO_TYPES = new Set([
    FileType.THUMBNAIL,
    FileType.CHAT_PHOTO,
    FileType.PHOTO,
    FileType.WALLPAPER,
    FileType.ENCRYPTED_THUMBNAIL,
]);

const DOCUMENT_TYPES = new Set(Object.values(FileType));

for (const item of PHOTO_TYPES) {
    DOCUMENT_TYPES.delete(item);
}

// converting Telegram Bot Api file_id to Telegram Client Api media object
export function convertBotFileToMessageMedia(fileId: string, fileSize: number): Api.TypeMessageMedia {
    const decoded = rle_decode(b64_decode(fileId));
    const major = decoded[decoded.length - 1];
    const buffer = major < 4 ? decoded.slice(0, -1) : decoded.slice(0, -2);

    let bufferPosition = 0;
    let fileType = buffer.readInt32LE(bufferPosition);
    bufferPosition += 4;
    const dcId = buffer.readInt32LE(bufferPosition);
    bufferPosition += 4;

    const hasWebLocation = Boolean(fileType & WEB_LOCATION_FLAG);
    const hasFileReference = Boolean(fileType & FILE_REFERENCE_FLAG);

    fileType &= ~WEB_LOCATION_FLAG;
    fileType &= ~FILE_REFERENCE_FLAG;

    if (!(fileType in FileType)) {
        throw new Error(`Unknown file_type ${fileType} of file_id ${fileId}`);
    }

    if (hasWebLocation) {
        const { result: url, newPosition } = readString(buffer, bufferPosition);
        bufferPosition = newPosition;

        const accessHash = buffer.readBigInt64LE(bufferPosition);
        bufferPosition += 8;

        // Fake type to return url
        const webpage = new Api.WebPage({
            id: bigInt(accessHash),
            hash: Number(accessHash.toString),
            url: url,
            displayUrl: url,
            attributes: [],
        });

        return new Api.MessageMediaWebPage({
            webpage: webpage,
        });
    }

    let fileReference: Buffer;
    if (hasFileReference) {
        const { result, newPosition } = readBytes(buffer, bufferPosition);
        fileReference = result;
        bufferPosition = newPosition;
    } else {
        fileReference = Buffer.alloc(0);
    }

    const media_id = BigInt(buffer.readBigInt64LE(bufferPosition).toString());
    bufferPosition += 8;
    const access_hash = BigInt(buffer.readBigInt64LE(bufferPosition).toString());
    bufferPosition += 8;

    if (PHOTO_TYPES.has(fileType)) {
        const photo = new Api.Photo({
            id: bigInt(media_id),
            accessHash: bigInt(access_hash),
            fileReference: fileReference,
            dcId: dcId,
            date: 0,
            sizes: [],
        });

        return new Api.MessageMediaPhoto({
            photo: photo,
        });
    }

    const document = new Api.Document({
        id: bigInt(media_id),
        accessHash: bigInt(access_hash),
        mimeType: "",
        date: 0,
        size: bigInt(fileSize),
        dcId: dcId,
        fileReference: fileReference,
        attributes: [],
    });

    return new Api.MessageMediaDocument({
        document: document,
    });
}

// converting Telegram Bot Api file_id to Telegram Client Api media object
export function extractMediaId(fileId: string): number {
    const decoded = rle_decode(b64_decode(fileId));
    const major = decoded[decoded.length - 1];
    const buffer = major < 4 ? decoded.slice(0, -1) : decoded.slice(0, -2);

    let bufferPosition = 0;
    let fileType = buffer.readInt32LE(bufferPosition);
    bufferPosition += 4;
    buffer.readInt32LE(bufferPosition);
    bufferPosition += 4;

    const hasFileReference = Boolean(fileType & FILE_REFERENCE_FLAG);

    fileType &= ~WEB_LOCATION_FLAG;
    fileType &= ~FILE_REFERENCE_FLAG;

    if (!(fileType in FileType)) {
        throw new Error(`Unknown file_type ${fileType} of file_id ${fileId}`);
    }

    if (hasFileReference) {
        const { newPosition } = readBytes(buffer, bufferPosition);
        bufferPosition = newPosition;
    }

    const mediaId = Number(buffer.readBigInt64LE(bufferPosition).toString());
    return mediaId;
}

function b64_decode(s: string): Buffer {
    const base64Padded = s + "=".repeat(mod(-s.length, 4));
    return Buffer.from(base64Padded, "base64");
}

function rle_decode(s: Buffer): Buffer {
    const r: number[] = [];
    let z = false;

    for (let i = 0; i < s.length; i++) {
        const b = s[i];

        if (!b) {
            z = true;
            continue;
        }

        if (z) {
            r.push(...Array(b).fill(0));
            z = false;
        } else {
            r.push(b);
        }
    }

    return Buffer.from(r);
}

function readBytes(buffer: Buffer, position: number): { result: Buffer; newPosition: number } {
    let length = buffer.readUInt8(position);
    position += 1;
    let padding = 0;

    if (length > 253) {
        length = buffer.readUIntLE(position, 3);
        position += 3;
        padding = mod(-length, 4);
    } else {
        padding = mod(-(length + 1), 4);
    }

    const result = buffer.slice(position, position + length);
    position += length + padding;
    return { result, newPosition: position };
}

function readString(buffer: Buffer, position: number): { result: string; newPosition: number } {
    const { result, newPosition } = readBytes(buffer, position);
    return { result: result.toString("utf8"), newPosition };
}

function mod(n: number, m: number): number {
    return ((n % m) + m) % m;
}
gram-js / gramjs

Here is my code to convert bot file id to media object to download big files using GramJs. You can integrate it. #554
