Why are some attachment file types not previewed in Fluid?

We recently implemented fluid expense reports and are reviewing input from users. While the attachment preview was a great feature, at this time most of our file types don't preview. While we didn't try all file types, generally we found that image files were previewed successfully and document type (PDF, DOC/X, XLS/X) were not. Is this common for all system users, or is this something that we have set up incorrectly? If this is not a feature we will put in a idea for it, as it seems straight forward. Our software administrators indicates some security concerns, but for us