OpenSSL Decryption using a Key.txt and IV.txt file - ssl

Question

Working with a client to set up OpenSSL file encryption. They've sent us an encrypted file (I'll call it sample.encrypted), along with key and iv files (key.txt and iv.txt).
The contents of the key.txt file look like this:
KEY: [string of 32 characters]
The contents of the iv.txt file look like this:
IV: [string of 16 characters]
I'm running Windows 7 Professional 64-bit, and learned that OpenSSL doesn't come installed by default with Windows (apparently it's primarily used by Linux users?)
So, I've downloaded and installed a 64-bit OpenSSL package from here:
(https:)//slproweb.com/products/Win32OpenSSL.html
Specifically, the "Win64 OpenSSL v1.0.2d Light" build found here:
(https:)//slproweb.com/download/Win64OpenSSL_Light-1_0_2d.exe
Once installed, I had to manually configure my environment variable for the OpenSSL config files as such (from the command prompt):
set OPENSSL_CONF=C:\OpenSSL-Win64\bin\openssl.cfg
I verified that the path information above was correct, and that the environment variable had stuck, and then attempted to run the following command:
openssl enc -d -K key.txt -iv iv.txt -in sample.encrypted -out sample.decrypted
This was based on the commands found here:https://www.openssl.org/docs/manmaster/apps/openssl.htmland here: https://www.openssl.org/docs/manmaster/apps/enc.html
The command breakdown being:
openssl - the OpenSSL base command
enc - the command used to begin encrypting/decrypting
-d - the tag used to specify decryption
-K - the tag used to specify the use of a key file
key.txt - the key file itself
-iv - the tag used to specify the use of an accompanying initialization vector
iv.txt - the IV file itself
-in - the tag used to specify the input file
sample.encrypted - the input file
-out - the tag used to specify the output file
sample.decrypted - the desired output file
As far as I can tell, the command works - the output file is generated, but its contents are not properly decrypted (it's just garbled text). I don't think there's anything wrong with the files that the client provided, but rather with my implementation of OpenSSL's commands to decrypt the file.
If anyone knows how to properly decrypt a file using provided Key and IV files, help would be greatly appreciated. I've been setting aside other responsibilities while Googling around trying to figure this out.