Those numbered files could just be random config files used by some app. If they were your media files that your copied over the names, and extensions, wouldn't change. You might be able to determine more by just looking at their file size. With the My Files app, long press on one those numbered files, and in the contextual menu that pops up select 'Details' and the file size will show.
(trivia - copying/moving files in Android will retain the same name, Apple and iTunes might be what you're thinking of. In order to prevent a user from being able to manipulate files in their own media file library, iTunes applies a coded I.D. to files, so when you view them using the Mac Finder the files names aren't recognizable as is. The conversion from actual name to coded name, or vice-versa when applicable, is stored within iTunes, a good incentive on Apple's part so you're always conditioned to just let iTunes maintain your media stuff.)
Getting back to your card issue, that doesn't bode well if those numbered files are indeed some kind of representation of the files you were copying over. That represents some kind of failed process. Are they actually located in the folder you were copying your files into, or in a different location? If in another folder or the root level of the card they're probably related to some other app so it might be best to leave them alone.