Para verificar as extensões já liberadas

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | select -ExpandProperty allowedfiletypes

Para verificar as extensões bloqueadas

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | select -ExpandProperty blockedfiletypes

Remover extensão da lista de bloqueados: Ex.: xml

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | Set-OwaMailboxPolicy -BlockedFileTypes @{Remove = “.xml”}

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | Set-OwaMailboxPolicy -BlockedMimeTypes @{Remove = “text/xml”, “application/xml”}

Adicionar na lista de extensões permitidas

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | Set-OwaMailboxPolicy –AllowedFileTypes @{Add = “.xml”}

Get-OwaMailboxPolicy OwaMailboxPolicy-Default | Set-OwaMailboxPolicy -AllowedMimeTypes @{Add = “text/xml”, “application/xml”}

Pronto a extensão já está liberada para a politica OwaMailboxPolicy-Default

Caso queira bloquear basta substituir:  “–AllowedFileTypes @{Add = “.xml”} ” por “–BlockedFileTypes @{Add = “.xml”} “ e  “BlockedMimeTypes @{Remove = “text/xml”, “application/xml”} ” por  “AllowedMimeTypes@{Remove = “text/xml”, “application/xml”} ”

Créditos: http://brunofeliciano.com/2014/01/24/liberar-extenso-de-anexo-no-owa-wave15office-365/

Extra:

Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedFileTypes @{Remove = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -AllowedFileTypes @{Add = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedMimeTypes @{Remove = "text/xml", "application/xml”}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy –AllowedMimeTypes @{Add = "text/xml", "application/xml”}