diff --git a/includes/mail.inc b/includes/mail.inc
index a778ac5..a0b53d6 100644
--- a/includes/mail.inc
+++ b/includes/mail.inc
@@ -418,10 +418,24 @@ function drupal_html_to_text($string, $allowed_tags = NULL) {
  * Helper function for array_walk in drupal_wrap_mail().
  *
  * Wraps words on a single line.
+ *
+ * Note that we are skipping MIME content headers lines, because attached files,
+ * especially applications, could have long MIME types or long filenames which
+ * result in line length longer than the 77 characters limit and wrapping that
+ * line will break the e-mail format. E.g. attached file hello_drupal.docx will
+ * produce the following Content-Type:
+ * Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document; name="hello_drupal.docx"
  */
 function _drupal_wrap_mail_line(&$line, $key, $values) {
-  // Use soft-breaks only for purely quoted or unindented text.
-  $line = wordwrap($line, 77 - $values['length'], $values['soft'] ? "  \n" : "\n");
+  // Use soft-breaks only for purely quoted or unindented text. Do not break
+  // MIME headers which could be longer than 77 characters.
+  $line_is_mime_header = strpos($line, 'Content-Type: ') === 0
+    || strpos($line, 'Content-Transfer-Encoding: ') === 0
+    || strpos($line, 'Content-Disposition: ') === 0
+    || strpos($line, 'Content-Description: ') === 0;
+  if (!$line_is_mime_header) {
+    $line = wordwrap($line, 77 - $values['length'], $values['soft'] ? "  \n" : "\n");
+  }
   // Break really long words at the maximum width allowed.
   $line = wordwrap($line, 996 - $values['length'], $values['soft'] ? " \n" : "\n");
 }