If the purpose is to retrieve the original file, since you can already download the original from somewhere else, then you don't need to decrypt it.
If the purpose to somehow discover the decryption key so you can decrypt other encrypted files, then comes the question if the other files were unchanged before being encrypted.
If the purpose is solely a study of decryption, then why the randomware example?
I shouldn't have mentioned ransomware. It was the first thing that popped in my mind. The purpose was to discover the key. I was curious if this was possible. My understanding of encryption is to keep others from viewing a file. This, of course, would assume that the unauthorized viewer did not have access to the decrypted file, which makes sense. However, if you could correctly guess one file and then retrieve an unencrypted copy from somewhere, I was assuming that it might be possible to use it to find the key. I was unaware of the plain text attack that Olof mentioned. If this were possible, it would make encryption pretty much useless, so I can see why it is addressed.
This really was never a discussion about ransomware.