don't decode base64 non-text parts

gpgmymail

@@ -87,7 +87,7 @@ def decode_email(message: email.message.Message) -> email.message.Message:
         decoded_bytes = message.as_string().encode()
     # if email doesn't need decoding
     has_quopri = b'Content-Transfer-Encoding: quoted-printable' in decoded_bytes
-    has_base64 = b'Content-Transfer-Encoding: base64' in decoded_bytes
+    has_base64 = is_base64_text(message)  # we don't want to decode non-text
     if not (has_quopri or has_base64):
         return message
     decoded_bytes = quopri.decodestring(decoded_bytes)
@@ -117,7 +117,7 @@ def decode_email(message: email.message.Message) -> email.message.Message:
     ) -> bytes:
         """
         change decoded_bytes such that part is decoded if base64 (unchanged if
-        not)
+        not) and text (so will not decode base64 images etc)
 
         see usage below for examples
 
@@ -128,7 +128,8 @@ def decode_email(message: email.message.Message) -> email.message.Message:
                                      don't overwrite this
         :return: bytes of decoded_bytes with part decoded if base64
         """
-        if part.get("Content-Transfer-Encoding") == "base64":
+        if part.get("Content-Transfer-Encoding") == "base64" and \
+            part.get_content_maintype() == "text":
             b64_str = part.get_payload()
             # remove the boundary as we don't want to change this
             if most_recent_boundary:
@@ -197,6 +198,25 @@ def set_email_header(
     else:
         message.add_header(name, value)
 
+def is_base64_text(message: email.message.Message) -> bool:
+    """
+    Return whether or not there is base64-encoded text in a multipart
+    message, i.e. will be False if it's only e.g. images that are encoded as
+    base64.
+
+    :param message: the email Message to check
+    :return: True if there is a text part that's base64 encoded, False if not
+    """
+    if message.is_multipart():
+        for part in message.walk():
+            if part.get('Content-Transfer-Encoding') == "base64" and \
+                part.get_content_maintype() == "text":
+                return True
+        return False
+    else:
+        return message.get('Content-Transfer-Encoding') == "base64" and \
+            message.get_content_maintype() == "text"
+
 def encrypt(
         message: email.message.Message,
         recipients: typing.List[str],
@@ -236,7 +256,7 @@ def encrypt(
         if is_message_encrypted(message):
             return message.as_string()
         # bc i just have a bunch of issues w b64
-        if "Content-Transfer-Encoding: base64" in message.as_string():
+        if is_base64_text(message):
             return message.as_string()
 
     # make necessary changes to message