async function startUpload() {
    if (!uploadsEnabled) {
        showToast("Guest uploads are disabled.", "warning");
        return;
    }
    if (uploadLocked) {
        showToast("Upload already started. Press Clear to create another box.", "warning");
        return;
    }
    if (!files.length) {
        showWarningDialog("No files selected", "There are no files selected. Please select files to upload.");
        showToast("No files selected. Please select files to upload.", "warning");
        setStatus("No files selected");
        return;
    }
    if (hasQuotaError()) {
        showWarningDialog("Over maximum upload size", quotaWarningMessage() || "Over maximum upload size.");
        showToast("Over maximum upload size.", "error");
        return;
    }

    uploadLocked = true;
    setBoxOptionsLocked(true);
    if (el.fileInput) el.fileInput.disabled = true;
    el.dropzone?.classList.add("is-locked");
    setShareUrl("");
    files.forEach((item) => {
        item.loaded = 0;
        item.uploaded = false;
        item.failed = false;
        item.error = "";
    });
    completedImpactKeys = new Set();
    overallImpactDone = false;
    renderFiles();

    let completedCount = 0;
    const totalCount = files.length;
    const statusPrefix = () => `${completedCount}/${totalCount}`;
    setStatus(`${statusPrefix()} Uploading.`);
    animateUploadStatus(statusPrefix);

    try {
        const box = await createBox();
        setShareUrl(box.box_url);
        files.forEach((item, index) => {
            item.boxID = box.box_id;
            item.boxFile = box.files[index];
            item.displayName = item.boxFile?.name || item.displayName;
            const icon = item.row?.querySelector(".upload-file-icon");
            if (icon && item.boxFile?.thumbnail_path) {
                item.row.classList.add("has-thumbnail");
                icon.src = item.boxFile.thumbnail_path;
            } else if (icon && item.boxFile?.icon_path && !item.previewURL) {
                icon.src = item.boxFile.icon_path;
            }
        });

        const results = await Promise.allSettled(files.map((item) => uploadFile(item, () => { completedCount += 1; })));
        stopStatusAnimation();

        const failedCount = results.filter((result) => result.status === "rejected").length;
        if (failedCount > 0) {
            setStatus(`${completedCount}/${totalCount} uploaded, ${failedCount} failed`);
            showToast(`${failedCount} file${failedCount === 1 ? "" : "s"} failed. The share URL contains the successful files.`, "error");
            renderFiles();
            return;
        }

        setOverallProgress(100);
        setStatus(`${completedCount}/${totalCount} uploaded. Share URL created. Press Clear to start another upload.`);
        showToast("Upload complete. Share URL created.");
        renderFiles();
    } catch (error) {
        stopStatusAnimation();
        uploadLocked = false;
        setBoxOptionsLocked(false);
        if (el.fileInput) el.fileInput.disabled = !uploadsEnabled;
        el.dropzone?.classList.remove("is-locked");
        setShareUrl("");
        setStatus(error.message || "Upload failed");
        showToast(error.message || "Upload failed", "error");
        renderFiles();
    }
}