I want to know if this is posible, Using serial or ethernet?
Yes, it is.
I have already followed some tutorials about sending txt files from a pc to a arduino but i have stil not found a way to send files other that plain text files.
The serial functions and the functions to write data to an SD card can't tell the difference between a byte that contains a value that you might interpret as a letter and one that you wouldn't. In other words, there is absolutely no difference between sending an mp3 file and sending a text file, except that the mp3 file can contain NULLs and the text file can't. Sending a NULL with Serial.print() isn't possible, but sending it using Serial.write() is. Not that that matters, because you won't be using Serial.print() or Serial.write() to send the data.