too late - you missed the needed file...
When you use the "Create Installation Media for Windows 8.1" from Microsoft, a "*.TEMP" File is downloaded in background.
This "*.TEMP" File is converted into "WINPAYLOAD.ESD" and right after the "Checking The Download" decryption process it is renamed "WINPAYLOAD.ESD.DECRYPT"
This is the decrypted windows installation package.. "WINPAYLOAD.ESD.DECRYPT" will be found in the following location
"C:\Users\your username\AppData\Local\Temp\yada yada some random alphanumeric\
\MediaCreationToolExpanded\WindowsInstallationMediaCreationTool\Download Folder"
Note: To view the AppData folder, you must first go into control panel, folder options, view tab, and select show hidden files, folders, and drives
You must copy the "WINPAYLOAD.ESD.DECRYPT" as soon as you see the tool finishes "checking the download", to your desktop, as this file will be deleted during the media tools media creation process