It does not let me upload my .tsx files after selecting the .json to use.
The tool currently only reads JSON-based Tiled files (.json, .tsj). I haven't implemented .tsx support yet.
Can Tiled convert your .tsx tilesets to .tsj? If that works, you should be able to use them with the current version. If native .tsx support is required, I'll need to set aside some time to implement and test it in a future update.
It can, but it's a bit of a pain in the butt to do it that way when you already have a pile of TSX tilesets. If you do decide to implement TSX support, that'd be great. As it is, my game loads TMX/TSX files, so I'll have to convert the existing TSXs to TSJ, then use your tool, then convert the JSONs back to Tiled format.