I am not sure what @Rh_Jaffery had in mind with “Upload feature”. The closest topic that I am aware of is to use /File. This will import a copy of the file (any file) into Coda, but it retains its original format.
You can simply have a link to the original file, or show an inline version of the file. If you want to edit the file, you still need to open it in the original tool used to create it.