Unable to find type System.Web.MimeMapping error - Powershell on Azure DevOps Pipeline

Question

I'm trying to run a Powershell script as a part of my build pipeline on Azure DevOps, but I keep getting the following error:

Unable to find type [System.Web.MimeMapping].
At D:\a\1\s\upload.ps1:31 char:15
 + $sourceMime = [System.Web.MimeMapping]::GetMimeMapping($sourceItem.Fu …
 +               ~~~~~~~~~~~~~~~~~~~~~~~~
 + CategoryInfo          : InvalidOperation: (System.Web.MimeMapping:TypeName) [], ParentContainsErrorRecordException
FullyQualifiedErrorId : TypeNotFound

The related bit of code in my Powershell script that's throwing the error:

# Get the source file contents and details, encode in base64
$sourceItem = Get-Item $sourceFile
$sourceBase64 = [Convert]::ToBase64String([IO.File]::ReadAllBytes($sourceItem.FullName))
$sourceMime = [System.Web.MimeMapping]::GetMimeMapping($sourceItem.FullName)

Any Pipelines gurus out there that can help? Note: I've already tried using Add-Type -AssemblyName System.Web to no avail. This seems to be something related to the hosted agent (windows-latest) since the script runs fine on my local machine.

Thanks in advance.