Use Set-Content instead of Set-Item for file uploads

[stegm]

According to https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.management/set-item?view=powershell-6 Set-Item is not supported by the FileSystem provider and so should not be used bei the GridFS-Provider.

Set-Content should be used instead (see https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.management/set-content?view=powershell-6)