PDF attachments are changing to unusable DAT files
I have an system that is attempting to email invoices to clients. This system works great except for one client gets .dat file instead of the nicely generated .pdf files that everyone else gets. 

I have researched the problem and many things point to issues with Outlook sending emails(not using outlook) and splitting the files because it is to large(it isn't, we can manualy send the email fine.)

After the headers on both files is just lines of characters that don't mean anything to me.

the headers of the .dat files:
file1

     =?utf-8?B?Y2lCQmJIQm9ZU0JKYm5OMWJHRjBhVzl1Pz0NCiA9P3V0Zi04P0I/SUNZ?=\
    \
    
     =?utf-8?B?Z1YyRjBaWEp3Y205dlptbHVaeUF0SUVGc2NHaGhJRWx1YzNWc1lYUnBi?=\
    \
    
     =?utf-8?B?MjRnSmlCWFlYUmw/PQ0KID0/dXRmLTg/Qj9jbkJ5YjI5bWFXNW5JT0tB?=\
    \
    
     =?utf-8?B?a3lCVmJtbDJaWEp6WVd3Z1EybDBlU3dnS0RJd01USXRNRFF0TVRNcD89?=\
    \
     =?utf-8?B?DQogPT91dGYtOD9CP0xuQmtaZz09Pz0=?="
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment

file2

     =?utf-8?B?YUdFZ1NXNXpkV3hoZEdsdmJpQW1JRmRoPz0NCiA9P3V0Zi04P0I/ZEdW?=\
    \
    
     =?utf-8?B?eWNISnZiMlpwYm1jZ0xTQkJiSEJvWVNCSmJuTjFiR0YwYVc5dUlDWWdW?=\
    \
    
     =?utf-8?B?MkYwWlhKd2NtOXY/PQ0KID0/dXRmLTg/Qj9abWx1WnlEaWdKTWdWVzVw?=\
    \
    
     =?utf-8?B?ZG1WeWMyRnNJRU5wZEhrc0lDZ3lNREV5TFRBMExURXpLUzV3WkdZPT89?=\
    \
     =?utf-8?B??="
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment

Does anyone have any idea on why this is happening?
PDF attachments are changing to unusable DAT files
===

I have an system that is attempting to email invoices to clients. This system works great except for one client gets .dat file instead of the nicely generated .pdf files that everyone else gets. 

I have researched the problem and many things point to issues with Outlook sending emails(not using outlook) and splitting the files because it is to large(it isn't, we can manualy send the email fine.)

After the headers on both files is just lines of characters that don't mean anything to me.

the headers of the .dat files:
file1

     =?utf-8?B?Y2lCQmJIQm9ZU0JKYm5OMWJHRjBhVzl1Pz0NCiA9P3V0Zi04P0I/SUNZ?=\
    \
    
     =?utf-8?B?Z1YyRjBaWEp3Y205dlptbHVaeUF0SUVGc2NHaGhJRWx1YzNWc1lYUnBi?=\
    \
    
     =?utf-8?B?MjRnSmlCWFlYUmw/PQ0KID0/dXRmLTg/Qj9jbkJ5YjI5bWFXNW5JT0tB?=\
    \
    
     =?utf-8?B?a3lCVmJtbDJaWEp6WVd3Z1EybDBlU3dnS0RJd01USXRNRFF0TVRNcD89?=\
    \
     =?utf-8?B?DQogPT91dGYtOD9CP0xuQmtaZz09Pz0=?="
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment

file2

     =?utf-8?B?YUdFZ1NXNXpkV3hoZEdsdmJpQW1JRmRoPz0NCiA9P3V0Zi04P0I/ZEdW?=\
    \
    
     =?utf-8?B?eWNISnZiMlpwYm1jZ0xTQkJiSEJvWVNCSmJuTjFiR0YwYVc5dUlDWWdW?=\
    \
    
     =?utf-8?B?MkYwWlhKd2NtOXY/PQ0KID0/dXRmLTg/Qj9abWx1WnlEaWdKTWdWVzVw?=\
    \
    
     =?utf-8?B?ZG1WeWMyRnNJRU5wZEhrc0lDZ3lNREV5TFRBMExURXpLUzV3WkdZPT89?=\
    \
     =?utf-8?B??="
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment

Does anyone have any idea on why this is happening?