extracting pdf attachments

I wrote a program to pull all the attachment from a CRM 2013 instance.  I think the code is typical:

        /// <summary>
        /// Takes an attachment and writes out the document to an existing directory named for the objectid
        /// </summary>
        /// <param name="att">an activitymimeattachment</param>
        static void writeAttachment(Entity att) {
            if (att.Contains("body")) {
                string filepath = string.Format(@".\{0}\{1}", att.GetAttributeValue<EntityReference>("objectid").Id.ToString()
                                                            , att.GetAttributeValue<string>("filename"));

                using (FileStream fs = new FileStream(filepath, FileMode.Create)) {
                    byte[] attContent = new UTF8Encoding(true).GetBytes(att.GetAttributeValue<string>("body"));
                    fs.Write(attContent, 0, attContent.Length);
                }
            }
        }

I was spot checking and discovered that any pdf I tried to open with Acrobat Reader came with the error message, "...could not open, ... because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded)."

I know that the way they got into CRM was by tracking the email via the Outlook client.  Hence, the example caught my attention.

What gives?

many thanks,