The below attachment types are blocked in the hosted version of Outlook for security and PCI-DSS compliancy reasons.   If you wish to open one of the below attachment types, please minimize your hosted products and open them in your locally installed Outlook.

.doc
.docx
.docm
.dot
.dotx
.dotm
.xls
.xlsx
.xlsm
.xlsb
.xml
.mht
.mhtml
.zip
.arc
.rar