Resume uploading with built-in Windows FTP client

No, the Windows command-line ftp.exe does not support transfer resuming.

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But you can just automatically download any small 3rd party portable command-line FTP client that supports automatic resume and use that.

For example the following PowerShell code downloads WinSCP .NET assembly package, extracts it and starts a resumable upload:

$winscp_assembly = "WinSCPnet.dll"
if (Test-Path $winscp_assembly)
{
    Write-Host "WinSCP already downloaded"
}
else
{
    $webclient = New-Object System.Net.WebClient
    $winscp_version = "5.17.9"
    $winscp_archive = "WinSCP-$winscp_version-Automation.zip"
    Write-Host "Downloading $winscp_archive ..."
    $url =
        "https://sourceforge.net/projects/winscp/files/WinSCP/" +
        $winscp_version + "/" + $winscp_archive + "/download"
    $webclient.DownloadFile($url, $winscp_archive)
    Write-Host "Done"

    Write-Host "Extracting $winscp_archive ..."
    $shell = New-Object -ComObject Shell.Application    

    $current_path = [string](Resolve-Path ".")
    $winscp_archive_path = [string](Resolve-Path $winscp_archive)
    $winscp_archive_folder = $shell.NameSpace($winscp_archive_path)
    $current_folder = $shell.NameSpace($current_path)
    $copy_options = 4 -bor 16 # SHCONTCH_NOPROGRESSBOX or SHCONTCH_RESPONDYESTOALL
    $current_folder.CopyHere($winscp_archive_folder.Items(), $copy_options)
    Write-Host "Done"
}

Add-Type -Path $winscp_assembly

$ftp_host = "ftp.example.com"
$ftp_path = "/target/path/"
$upload_path = "C:bigfile.dat"
Write-Host "Starting resumable upload of $upload_path to $ftp_host ..."
# Setup session options
$sessionOptions = New-Object WinSCP.SessionOptions
$sessionOptions = New-Object WinSCP.SessionOptions -Property @{
    Protocol = [WinSCP.Protocol]::Ftp
    HostName = $ftp_host
    UserName = "username"
    Password = "password"
}

$session = New-Object WinSCP.Session
$session.Open($sessionOptions)

$session.PutFiles($upload_path, $ftp_path).Check()

To run the PowerShell script (upload.ps1) use:

powershell.exe -File upload.ps1 -ExecutionPolicy Bypass

(I'm the author of WinSCP)

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Another option is to implement the resume manually using FtpWebRequest.

See How to continue or resume FTP upload after interruption of internet.

Again you can use the FtpWebRequest from a PowerShell script. See Upload files with FTP using PowerShell.