Converting a string rappresentation of a file (byte array) back to a file in C#

Converting a string rappresentation of a file (byte array) back to a file
in C#

As in the title I'm trying to convert back a string rappresentation of a
bytearray to the original file where the bytes where taken.
What I've done:
I've a web service that gets a whole file and sends it:
answer.FileByte = File.ReadAllBytes(@"C:\QRY.txt");
After the serialization in the transmitted result xml I've this line:
<a:FileByte>TVNIfGF8MjAxMzAxMDF8YQ1QSUR8YXxhfGF8YXxhfGF8YXwyMDEzMDEwMXxhfGF8YXxhfGF8YXxhfGF8YXxhfGF8YXxhDVBWMXxhfGF8YXxhfGF8YXxhfGF8YXxhfDIwMTMwMTAxfDIwMTMwMTAxfDB8MHxhDQo=</a:FileByte>
I've tried to convert it back with this line in another simple application:
//filepath is the path of the file created
//bytearray is the string from the xml (copypasted)
File.WriteAllBytes(filepath, Encoding.UTF8.GetBytes(bytearray));
I've used UTF8 as enconding since the xml declares to use this charset.
Keeping the datatype is not an option since I'm writing a simple utility
to check the file conversion.
Maybe I'm missing something very basic but I'm not able to come up with a
working solution.