The cause appears to be the use of non-ASCII characters in file names. Microsoft describes it like so: “Any user may be ...
Microsoft has started looking into a problem that is preventing users from opening some Excel file attachments with non-ASCII ...
XDA Developers on MSN
Sorry, the new Outlook app can't open some Excel attachments, says Microsoft
There is a workaround, though.
Double-clicking on a .XLS file will start Excel, but will not cause the file to open. I've been through the File Associations, made sure that DDE was selected and had the right parameters, and it ...
Microsoft is working to resolve a known issue that prevents some users from opening Excel email attachments in the new Outlook client.
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