If I wanted to brute-force decrypt data, I would try decrypting it with a key and see if the decryption returned nonsense. If the decryption returned meaningful data (ex. something that a natural language processor deemed English, if I was trying to decrypt an English language email), I would assume I had the key. If it did not return meaningful data, I would try again with the next possible key until I had tried all possible keys. But this method would not work if I did not know if I was trying to decrypt a picture, text in Spanish, text in Chinese, an audio file etc.
How do you brute force decrypt if you don't know what you are trying to find?