I cannot duplicate your problem. There has to be something else going on.
Please show us the following:
Can you ftp (binary copy) the xls file somewhere, or copy it to a usb drive, so you can then open it on your PC? -- need to know if the file itself is okay; the only way is to do the above copy. Please DO NOT assume your .xls file on the unix box is okay. It probably is not. It probably has UNIX carriage control, which excel may not like very much.
In every DOS system, the file extensions alone identify the file type. These are internally associated with the appropriate programme to open the file.
In your webmail (assuming its a client), check the association for xls. If none, see whether it defaults to using local associations.
For an experiment, in the PC where you opened outlook, use mycomputer --> tools --> folderoptions --> filetypes and delete the association for XLS.