message: improve attachment heuristics

Do not consider calender-invitations "attachments"; do mark as
"calendar". Do recognize application/ics as calendar messages.

Update mime-object to expose a message part's disposition.

Change the "is-attachment" heuristic to include inline parts if they
have a filename parameter in their content-disposition.

Note that this doesn't change things radically; the delta is +69 and
-202 for ~6500 attachments.

5 files changed, 71 insertions, 35 deletions

diff --git a/lib/message/mu-message-part.cc b/lib/message/mu-message-part.cc
index d1c7ac5..3ed0b9c 100644
--- a/lib/message/mu-message-part.cc
+++ b/lib/message/mu-message-part.cc
@@ -195,17 +195,22 @@ MessagePart::looks_like_attachment() const noexcept
 	if (!ctype)
 		return false; // no content-type: not an attachment.
 
-	// we consider some parts _not_ to be attachments regardless of disposition
-	if  (matches(*ctype,{{"application", "pgp-keys"}}))
+	// we consider some parts _not_ to be attachments regardless of
+	// disposition
+	if  (matches(*ctype,{{"application", "pgp-keys"},
+			     {"application", "ics"}}))
 		return false;
 
-	// we consider some parts to be attachments regardless of disposition
-	if  (matches(*ctype,{{"image", "*"},
-			     {"audio", "*"},
-			     {"application", "*"},
-			     {"application", "x-patch"}}))
+	if (is_attachment()) // i.e., as per content-disposition
 		return true;
 
+	// we also consider "inline" parts as attachment, if the
+	// content-disposition has a filename property.
+	if (const auto cdisp{mime_object().content_disposition()}; !!cdisp) {
+		if (const auto& fname{cdisp->parameter("filename")}; !!fname)
+			return true;
+	}
+
 	// otherwise, rely on the disposition
 	return is_attachment();
 }
diff --git a/lib/message/mu-message.cc b/lib/message/mu-message.cc
index 126e8df..ebe3f81 100644
--- a/lib/message/mu-message.cc
+++ b/lib/message/mu-message.cc
@@ -424,9 +424,11 @@ process_part(const MimeObject& parent, const MimePart& part,
 	if (!ctype)
 		return;
 
-	// flag as calendar, if not already
+	// flag as calendar, if not already.
 	if (none_of(info.flags & Flags::Calendar) &&
-	    ctype->is_type("text", "calendar"))
+	    (ctype->is_type("text", "calendar") ||
+	     ctype->is_type("text", "x-vcalendar") ||
+	     ctype->is_type("application", "ics")))
 		info.flags |= Flags::Calendar;
 
 	// flag as attachment, if not already.
diff --git a/lib/message/mu-mime-object.hh b/lib/message/mu-mime-object.hh
index 428c399..6c846a3 100644
--- a/lib/message/mu-mime-object.hh
+++ b/lib/message/mu-mime-object.hh
@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2022-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2022-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -171,7 +171,6 @@ private:
 };
 
 
-
 /**
  * Thin wrapper around a GMimeContentType
  *
@@ -197,7 +196,6 @@ struct MimeContentType: public Object {
 		return g_mime_content_type_is_type(self(), type.c_str(),
 						   subtype.c_str());
 	}
-
 	Option<std::string> parameter(const std::string& name) const noexcept {
 		const char *param{g_mime_content_type_get_parameter(self(), name.c_str())};
 		if (!param || !param[0])
@@ -205,17 +203,47 @@ struct MimeContentType: public Object {
 		else
 			return Some(std::string{param});
 	}
-
 private:
 	GMimeContentType* self() const {
 		return reinterpret_cast<GMimeContentType*>(object());
 	}
 };
 
+/**
+ * Thin wrapper around a GMimeContentDisposition
+ *
+ */
+struct MimeContentDisposition: public Object {
+
+	MimeContentDisposition(GMimeContentDisposition *disp) : Object{G_OBJECT(disp)} {
+		if (!GMIME_IS_CONTENT_DISPOSITION(self()))
+			throw std::runtime_error("not a content-disposition");
+	}
 
+	std::string disposition() const noexcept {
+		if (const auto disp{g_mime_content_disposition_get_disposition(self())}; disp)
+			return disp;
+		else
+			return {};
+	}
+	bool is_attachment() const noexcept {
+		return g_mime_content_disposition_is_attachment(self());
+	}
+
+	Option<std::string> parameter(const std::string& name) const noexcept {
+		const char *param{g_mime_content_disposition_get_parameter(self(), name.c_str())};
+		if (!param || !param[0])
+			return Nothing;
+		else
+			return Some(std::string{param});
+	}
+private:
+	GMimeContentDisposition* self() const {
+		return reinterpret_cast<GMimeContentDisposition*>(object());
+	}
+};
 
 
-
 /**
  * Thin wrapper around a GMimeStream
  *
@@ -272,7 +300,6 @@ constexpr Option<std::string_view> to_string_view_opt(const S& seq, T t) {
 		return it->second;
 }
 
-
 /**
  * Thin wrapper around a GMimeDataWrapper
  *
@@ -298,7 +325,6 @@ private:
 };
 
 
-
 /**
  * Thin wrapper around a GMimeCertifcate
  *
@@ -494,7 +520,6 @@ constexpr Option<std::string_view> to_string_view_opt(MimeCertificate::Validity
 }
 
 
-
 /**
  * Thin wrapper around a GMimeSignature
  *
@@ -575,7 +600,6 @@ static inline std::string to_string(MimeSignature::Status status) {
 
 
 
-
 /**
 * Thin wrapper around a GMimeDecryptResult
  *
@@ -645,7 +669,6 @@ constexpr Option<std::string_view> to_string_view_opt(MimeDecryptResult::CipherA
 	return to_string_view_opt(AllCipherAlgos, algo);
 }
 
-
 /**
  * Thin wrapper around a GMimeCryptoContext
  *
@@ -748,7 +771,6 @@ private:
 	}
 };
 
-
 /**
  * Thin wrapper around a GMimeObject
  *
@@ -820,6 +842,19 @@ public:
 	}
 
 	/**
+	 * Get the content disposition
+	 *
+	 * @return the content-disposition or Nothing
+	 */
+	Option<MimeContentDisposition> content_disposition() const noexcept {
+		auto disp{g_mime_object_get_content_disposition(self())};
+		if (!disp)
+			return Nothing;
+		else
+			return MimeContentDisposition(disp);
+	}
+
+	/**
 	 * Write this MimeObject to some stream
 	 *
 	 * @param f_opts formatting options
@@ -918,7 +953,6 @@ private:
 	}
 };
 
-
 /**
  * Thin wrapper around a GMimeMessage
  *
@@ -1024,7 +1058,6 @@ private:
 		return reinterpret_cast<GMimeMessage*>(object());
 	}
 };
-
 /**
  * Thin wrapper around a GMimePart.
  *
@@ -1200,7 +1233,6 @@ private:
 };
 
 
-
 /**
  * Thin wrapper around a GMimeMessagePart.
  *
@@ -1235,7 +1267,7 @@ private:
 	}
 
 };
-/**
+/**
  * Thin wrapper around a GMimeApplicationPkcs7Mime
  *
  */
@@ -1270,7 +1302,6 @@ private:
 	}
 };
 
-
 /**
  * Thin wrapper around a GMimeMultiPart
  *
@@ -1327,7 +1358,6 @@ private:
 	}
 };
 
-
 /**
  * Thin wrapper around a GMimeMultiPartEncrypted
  *
@@ -1365,7 +1395,6 @@ private:
 
 MU_ENABLE_BITOPS(MimeMultipartEncrypted::DecryptFlags);
 
-
 /**
  * Thin wrapper around a GMimeMultiPartSigned
  *
@@ -1388,8 +1417,6 @@ public:
 		EnableOnlineCertificateChecks = GMIME_VERIFY_ENABLE_ONLINE_CERTIFICATE_CHECKS
 	};
 
-	// Result<std::vector<MimeSignature>> verify(VerifyFlags vflags=VerifyFlags::None) const noexcept;
-
 	Result<std::vector<MimeSignature>> verify(const MimeCryptoContext& ctx,
 						  VerifyFlags vflags=VerifyFlags::None) const noexcept;
 
diff --git a/lib/message/test-mu-message.cc b/lib/message/test-mu-message.cc
index 1e365f7..c562ba2 100644
--- a/lib/message/test-mu-message.cc
+++ b/lib/message/test-mu-message.cc
@@ -814,8 +814,7 @@ RU5EOlZFVkVOVA0KRU5EOlZDQUxFTkRBUg0K
 	g_assert_true(!!message);
 	assert_equal(message->subject(),
 		     "Invitation: HELLO, @ Thu 9 Jan 2014 08:30 - 09:30 (william@example.com)");
-	g_assert_true(message->flags() == (Flags::Passed|Flags::Seen|
-					   Flags::HasAttachment|Flags::Calendar));
+	g_assert_true(message->flags() == (Flags::Passed|Flags::Seen|Flags::Calendar));
 	g_assert_cmpuint(message->body_html().value_or("").find("DETAILS"), ==, 2271);
 }
 
diff --git a/mu4e/mu4e.texi b/mu4e/mu4e.texi
index e1ab6ed..383d975 100644
--- a/mu4e/mu4e.texi
+++ b/mu4e/mu4e.texi
@@ -1602,7 +1602,7 @@ mu4e-view-show-mime-parts}. This can be a little slow.
 
 Nowadays, typical e-mail messages can be thought of as a series of
 ``MIME-parts'', which are sections of the message. The most prominent of those
-parts is the 'body', which is the main text of the message your are readings.
+parts is the 'body', which is the main text of the message your are reading.
 
 Other MIME-parts in the messages include @emph{attachments}.
 
@@ -1610,9 +1610,12 @@ Other MIME-parts in the messages include @emph{attachments}.
 @cindex attachments
 
 Many e-mail messages contain @emph{attachments}, which are MIME-parts that
-encode files@footnote{Attachments come in two flavors: @t{inline} and
-@t{attachment}. @code{mu4e} does not distinguish between the two when operating on
-them: everything that specifies a filename is considered an attachment}.
+encode files which you can extract.
+
+@footnote{@code{mu} uses some heuristics to decide if a part should be treated
+as an attachment; including @t{inline} parts that specify a filename. The
+heuristic tries to balance false-positive and false-negatives, both of which are
+possible}
 
 To save attachments as files on your computer, @code{mu4e}'s message-view
 offers the command @code{mu4e-view-save-attachments}; its default keybinding is