The headers already sent error is caused by output between PHP tags. it could be a blank space, a blank line or HTML code.
this section of the error:
headers already sent by (output started at /home/content/69/10142169/html/send_file.php:37)
is telling you that the problem is caused around line 37 of the send_file.php file. look for a closing php tag at line 36 and any out put after it.
see this thread for more details on the cause of the headers already sent error and how to troubleshoot: