I have an encrypted file and I forgot the decryption key, how can I decrypt the file?

I suggest you forget about restoring your encrypted file in this case. Decryption key is not like password which we set and remember, instead it is extremely random long sequence of characters which is made by system and what you remember (the general gist) would be the passphrase to unlock that key (something like password to access the key). If you have lost the key than its sad that you cannot decrypt it unless you made the backup of your key and have it safe somewhere. Even if you somehow find a tool or develop your own to brute force the key then, depending on encryption algorithm, mode and keysize used to encrypt the file it will take some thousand of years to do so (nearly impossible).