CrushFTP by default does not have support for inline docs and sheets and slides editing, but you can try the below steps, for using the chrome builtin extensions.

In your WebInterface/mime_types.txt file, add these: 
{{{.doc application/msword 
.docx application/msword 
.xls application/ 
.xlsx application/}}}

Then in your UserManager, WebInterface, Customizations...enable the extension for opens in new window. Add "doc,docx,xls,xslx"

Now clicking these types will open another tab for viewing the document. This was the only extension I found that actually functions...the rest were just gimmicks to give people a shortcut link to take them to third party sites.

If they don't have the extension enabled, clicking the file will just download it regardless of your config. If they have the extension, it will open and view it in the browser.

The browser will cache download files, so if you try this before having the extension installed, it will just keep downloading the file regardless of what you try until emptying cache.