Have you heard of magic numbers? Some file formats are designed such that files are always saved with a specific byte sequence in the header. JPEG, PDF, and ZIP are all such formats.

You could look for ZIP files by searching for all files with a .zip extension but a better way would be to look for all files that have 50 4b 03 04 as the first 4 bytes of the file. All ZIP files will start with those bytes. Not all ZIP files have the .zip extension.

Here’s a Test-ZipFile PowerShell cmdlet that will return true or false whether the specified file has this magic header. You may also note that this cmdlet is a good citizen by accepting file path input in an idiomatic way.

This cmdlet is also a great example of accepting -Path and -LiteralPath arguments in an idiomatic way. Including wildcard support for -Path.

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function Test-ZipFile
{
<#
.SYNOPSIS
    Tests for the magic ZIP file header bytes.

.DESCRIPTION
    Inspired by http://stackoverflow.com/a/1887113/31308
#>
	[CmdletBinding()]
	param(
        [Parameter(
            ParameterSetName  = "Path",
            Mandatory = $true,
            ValueFromPipeline = $true,
            ValueFromPipelineByPropertyName = $true
        )]
        [string[]]$Path,

        [Alias("PSPath")]
        [Parameter(
            ParameterSetName = "LiteralPath",
            Mandatory = $true,
            ValueFromPipeline = $true,
            ValueFromPipelineByPropertyName = $true
        )]
        [string[]]$LiteralPath
	)

    process {
        $provider = $null

        # Only expand wildcards if the -Path parameter was used.
        if ($PSCmdlet.ParameterSetName -eq "Path") {
            $filePaths = $PSCmdlet.GetResolvedProviderPathFromPSPath($Path, [ref]$provider)
        }
        elseif ($PSCmdlet.ParameterSetName -eq "LiteralPath") {
            $filePaths = $PSCmdlet.GetResolvedProviderPathFromPSPath($LiteralPath, [ref]$provider)
        }

        foreach ($filePath in $filePaths) {
            $isZip = $false
            try {
                $stream = New-Object System.IO.StreamReader -ArgumentList @($filePath)
                $reader = New-Object System.IO.BinaryReader -ArgumentList @($stream.BaseStream)
                $bytes = $reader.ReadBytes(4)
                if ($bytes.Length -eq 4) {
                    if ($bytes[0] -eq 80 -and
                        $bytes[1] -eq 75 -and
                        $bytes[2] -eq 3 -and
                        $bytes[3] -eq 4) {
                        $isZip = $true
                    }
                }
            }
            finally {
                if ($reader) {
                    $reader.Dispose()
                }
                if ($stream) {
                    $stream.Dispose()
                }
            }

            Write-Output $isZip
        }
    }
}

Test-ZipFile is part of Poshato, my personal PowerShell module of miscellaneous goodness.