It is impossible to force a client to use Excel for any particular content type. But, as long as the content-type and content-disposition of the response is set properly, it is likely that Excel will be used to view the content.
A content-type of application/vnd.ms-excel will indicate that the response is Excel data. A content-disposition header should use a filename extension that is typically associated with Excel on Windows machines. Both are need since IE often ignores the content-type and uses the filename extension to determine which application to use.
You don’t want a string list. You’ll want one string. Probably the right thing to do is convert the string list to a string, using line terminators to separate each item in the list. Since you’ll be returning a string, you can use pub.flow:setResponse to set the response content and headers.
If you find you want/need to return bytes, refer to [URL]wmusers.com for a Java service that may work for you.
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