Hello Miker 866,
Welcome to Rootschat

. This photo probably dates from the the late 1870's. The French style kepi's had been around since the Crimean War and well known from the American Civil War.
You mention Plovdiv, there was a lot going on in 1877-1878, namely the Russo-Turkish War. The Russians captured Plovdiv and wanted to add it to Bulgaria. After all, Bulgarians made up most of the population. The British (and French?) vetoed it until the Treaty of Berlin, Disraeli and Bismarck.
I think your ancestor was in Bulgarian or associated allied uniform. He had a medal (Russo-Turkish War). I say 'associated' because his uniform does not match up exactly with Bulgarian uniforms.
I think you can discount Poland (famous last words

)