XSLT file picker in Umbraco Forms can't access files from media (when not using file system)

[AndyButland]

A brief description of the issue goes here.
Objects uploaded to the Media library in Umbraco Cloud (new infrastructure Umbraco 8) are inaccessible to the XSLT file media picker in Umbraco Forms.
This is preventing from using XSLT files to transform the XML before posting it to an API endpoint.

Reproduction
If you're filing a bug, please describe how to reproduce it. Include as much
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Bug summary
Specifics
This was tested on:
Umbraco: 8.17.2
Umbraco.Forms: 8.10.2

Mentioned before in:
#477 (comment)

Steps to reproduce

1. Log into any Umbraco 8 Umbraco Cloud site using the new infrastructure
2. Click "Media" from the navigation menu
3. Upload a test file like sample2.xslt
4. Click "Forms" from the navigation menu
5. Create new form or edit an existing form
6. Click "Configure Workflow"
7. Add the Workflow "Post as XML"
8. At "XSLT File" click "Pick" and select the XML file uploaded to the media folder
9. Save the form
10. Publish the form or visit the published form
11. Fill in the form and submit.
12. Browse to Settings -> Log viewer and check the errors

Exception
System.IO.DirectoryNotFoundException: Could not find a part of the path 'C:\home\site\wwwroot\media\325jzesq\sample2.xslt'.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
   at System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials, IWebProxy proxy, RequestCachePolicy cachePolicy)
   at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn)
   at System.Xml.Xsl.Xslt.XsltLoader.CreateReader(Uri uri, XmlResolver xmlResolver)
   at System.Xml.Xsl.Xslt.XsltLoader.Load(Compiler compiler, Object stylesheet, XmlResolver xmlResolver)
   at System.Xml.Xsl.Xslt.Compiler.Compile(Object stylesheet, XmlResolver xmlResolver, QilExpression& qil)
   at System.Xml.Xsl.XslCompiledTransform.LoadInternal(Object stylesheet, XsltSettings settings, XmlResolver stylesheetResolver)
   at Umbraco.Forms.Core.Data.Helpers.XsltHelper.TransformXML(String xml, String xsltFile)
   at Umbraco.Forms.Core.Providers.WorkflowTypes.SendXsltEmail.Execute(Record record, RecordEventArgs e)

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