m->d: Support attachment body data as alt text

m2d/converters/event-to-message.js

@@ -535,19 +535,22 @@ async function eventToMessage(event, guild, di) {
 		}
 	} else if (event.type === "m.room.message" && (event.content.msgtype === "m.file" || event.content.msgtype === "m.video" || event.content.msgtype === "m.audio" || event.content.msgtype === "m.image")) {
 		content = ""
-		const filename = event.content.body
+		const filename = event.content.filename || event.content.body
+		// A written `event.content.body` will be bridged to Discord's image `description` which is like alt text.
+		// Bridging as description rather than message content in order to match Matrix clients (Element, Neochat) which treat this as alt text or title text.
+		const description = (event.content.body !== event.content.filename && event.content.filename && event.content.body) || undefined
 		if ("url" in event.content) {
 			// Unencrypted
 			const url = mxUtils.getPublicUrlForMxc(event.content.url)
 			assert(url)
-			attachments.push({id: "0", filename})
+			attachments.push({id: "0", description, filename})
 			pendingFiles.push({name: filename, url})
 		} else {
 			// Encrypted
 			const url = mxUtils.getPublicUrlForMxc(event.content.file.url)
 			assert(url)
 			assert.equal(event.content.file.key.alg, "A256CTR")
-			attachments.push({id: "0", filename})
+			attachments.push({id: "0", description, filename})
 			pendingFiles.push({name: filename, url, key: event.content.file.key.k, iv: event.content.file.iv})
 		}
 	} else if (event.type === "m.sticker") {

m2d/converters/event-to-message.test.js

@@ -2143,7 +2143,7 @@ test("event2message: text attachments work", async t => {
 				username: "cadence [they]",
 				content: "",
 				avatar_url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/azCAhThKTojXSZJRoWwZmhvU",
-				attachments: [{id: "0", filename: "chiki-powerups.txt"}],
+				attachments: [{id: "0", description: undefined, filename: "chiki-powerups.txt"}],
 				pendingFiles: [{name: "chiki-powerups.txt", url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/zyThGlYQxvlvBVbVgKDDbiHH"}]
 			}]
 		}
@@ -2157,6 +2157,7 @@ test("event2message: image attachments work", async t => {
 			sender: "@cadence:cadence.moe",
 			content: {
 				body: "cool cat.png",
+				filename: "cool cat.png",
 				info: {
 					size: 43170,
 					mimetype: "image/png",
@@ -2178,7 +2179,43 @@ test("event2message: image attachments work", async t => {
 				username: "cadence [they]",
 				content: "",
 				avatar_url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/azCAhThKTojXSZJRoWwZmhvU",
-				attachments: [{id: "0", filename: "cool cat.png"}],
+				attachments: [{id: "0", description: undefined, filename: "cool cat.png"}],
+				pendingFiles: [{name: "cool cat.png", url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/IvxVJFLEuksCNnbojdSIeEvn"}]
+			}]
+		}
+	)
+})
+
+test("event2message: image attachments can have a custom description", async t => {
+	t.deepEqual(
+		await eventToMessage({
+			type: "m.room.message",
+			sender: "@cadence:cadence.moe",
+			content: {
+				body: "Cat emoji surrounded by pink hearts",
+				filename: "cool cat.png",
+				info: {
+					size: 43170,
+					mimetype: "image/png",
+					w: 480,
+					h: 480,
+					"xyz.amorgan.blurhash": "URTHsVaTpdj2eKZgkkkXp{pHl7feo@lSl9Z$"
+				},
+				msgtype: "m.image",
+				url: "mxc://cadence.moe/IvxVJFLEuksCNnbojdSIeEvn"
+			},
+			event_id: "$CXQy3Wmg1A-gL_xAesC1HQcQTEXwICLdSwwUx55FBTI",
+			room_id: "!BnKuBPCvyfOkhcUjEu:cadence.moe"
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "cadence [they]",
+				content: "",
+				avatar_url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/azCAhThKTojXSZJRoWwZmhvU",
+				attachments: [{id: "0", description: "Cat emoji surrounded by pink hearts", filename: "cool cat.png"}],
 				pendingFiles: [{name: "cool cat.png", url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/IvxVJFLEuksCNnbojdSIeEvn"}]
 			}]
 		}
@@ -2228,7 +2265,7 @@ test("event2message: encrypted image attachments work", async t => {
 				username: "cadence [they]",
 				content: "",
 				avatar_url: "https://matrix.cadence.moe/_matrix/media/r0/download/cadence.moe/azCAhThKTojXSZJRoWwZmhvU",
-				attachments: [{id: "0", filename: "image.png"}],
+				attachments: [{id: "0", description: undefined, filename: "image.png"}],
 				pendingFiles: [{
 					name: "image.png",
 					url: "https://matrix.cadence.moe/_matrix/media/r0/download/heyquark.com/LOGkUTlVFrqfiExlGZNgCJJX",

types.d.ts

@@ -91,6 +91,7 @@ export namespace Event {
 	export type M_Room_Message_File = {
 		msgtype: "m.file" | "m.image" | "m.video" | "m.audio"
 		body: string
+		filename?: string
 		url: string
 		info?: any
 		"m.relates_to"?: {
@@ -107,6 +108,7 @@ export namespace Event {
 	export type M_Room_Message_Encrypted_File = {
 		msgtype: "m.file" | "m.image" | "m.video" | "m.audio"
 		body: string
+		filename?: string
 		file: {
 			url: string
 			iv: string