From what I can tell, the only way to add files to the firmware repository is to login to the server hosting Solarwinds and add it directly to the folder that Solarwinds queries for firmware repository.
I feel it would be much easier from a workflow standpoint to have the option to add these files from inside the web console.
The way it's currently set up takes you directly out of the web console and into a server a user may or may not have access to, to then attempt to add a file to a folder the user, again, may or may not have access to. These permission concerns are taken care of within the console because the account querying the repository has already gone through the AAA process.
Basically, from Settings > NCM Settings > Firmware Repository (found under Firmware Ugrades)
Add a function to upload a file to the repository