fix: image/*, audio/*, video/* are not attachments when found in both text and html parts

RFC8621 section 4.1.4
The additional is_multipart heuristic prevents the tests for malformed .eml
from failing.

Author: sftse

src/parsers/message.rs

@@ -314,9 +314,10 @@ impl MessageParser {
                     message.text_body.push(message.parts.len());
                 }
 
-                let is_html = mime_type == MimeType::TextHtml;
-
-                if !is_text || !add_to_html && is_html || !add_to_text && !is_html {
+                if !is_multipart && !add_to_text && !add_to_html
+                    || (is_multipart || mime_type == MimeType::Inline)
+                        && (!add_to_text || !add_to_html)
+                {
                     message.attachments.push(message.parts.len());
                 }
 
@@ -338,6 +339,8 @@ impl MessageParser {
                         (Cow::Borrowed(bytes), None) => String::from_utf8_lossy(bytes),
                     };
 
+                    let is_html = mime_type == MimeType::TextHtml;
+
                     if is_html {
                         PartType::Html(text)
                     } else {