Test-Image.ps1

<#
.Synopsis
        Compares the first 8 bytes of a given file to a set of known image headers.
.Example
  	PS> dir .\1.tiff | Test-Image
    True

    PS> Test-Image -Path test.ps1
    False
#>
function Test-Image {
    [CmdletBinding()]
    [OutputType([System.Boolean])]
    param(
        [Parameter(Mandatory=$true, Position=0, ValueFromPipeline=$true)] 
        [ValidateNotNullOrEmpty()]
        [Alias('PSPath')]
        [string] $Path
    )

    PROCESS {
        $knownImageHeaders = @{
            jpg = @( "FF", "D8" );
            bmp = @( "42", "4D" );
            gif = @( "47", "49", "46" );
            tif = @( "49", "49", "2A" );
            png = @( "89", "50", "4E", "47", "0D", "0A", "1A", "0A" );
            pdf = @( "25", "50", "44", "46" );
        }

        $bytes = Get-Content -LiteralPath $path -Encoding Byte -ReadCount 1 -TotalCount 8 -ErrorAction Ignore

        $retval = $false
        foreach($key in $knownImageHeaders.Keys) {

            # transform into array of the same length and format as the reference array
            $byteArray = $bytes | Select-Object -First $knownImageHeaders[$key].Length | ForEach-Object { $_.ToString("X2") }
            if($byteArray.Length -eq 0) {
                continue
            }

            # compare the two arrays
            $diff = Compare-Object -ReferenceObject $knownImageHeaders[$key] -DifferenceObject $byteArray
            if(($diff | Measure-Object).Count -eq 0) {
                $retval = $true
            }
        }

        return $retval
    }
}

function Get-ChildImage {
    [CmdletBinding()]
    [OutputType([System.IO.FileInfo])]
    param(
        [parameter(Mandatory=$false, Position=0, ValueFromPipeline=$true)]
        [ValidateNotNullOrEmpty()]
        [Alias('PSPath')]
        [string] $Path
    )

    Get-ChildItem $Path -File | Where-Object { Test-Image $_.FullName }
}