I've created the workflow for you, which will show you how you can search inside files using a file filter workflow.
The keyword is for this file filter workflow is set to "exin" and allows you to search for keywords inside Excel files. For example, if I'm looking for the keyword "business", I will type "exin business".
You can download the workflow here:
https://dl.dropboxusercontent.com/u/5818788/Alfred/Excel%20Content.alfredworkflow
