ze58dadc2 is an randomly generated string, they can use a database (they fill in themselves) to look up what filename it belongs to, which would return
pchelka_mia. There is no magic algorithm going on here, just basic lookup, which is why you can't decode them.
They keep the extension intact because it's always four characters and they don't want to be storing (even though compressed) the extra four characters in the database. Furthermore, the reason they usually do this is to make URLs shorter and anonymize the picture for when you share it, such that people don't know the name of the person / album when they get their hands to it.
It's not stored in exif simply because there is no field for it (unless you misuse another field), and for the sake of simplicity they aren't going to manipulate the image when you upload it (apart from maybe stripping the exif data so they can't track your location).