Code block attachments use Discord supported types

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

@@ -894,7 +894,8 @@ async function eventToMessage(event, guild, channel, di) {
 						let preNode
 						if (node.nodeType === 3 && node.nodeValue.includes("```") && (preNode = nodeIsChildOf(node, ["PRE"]))) {
 							if (preNode.firstChild?.nodeName === "CODE") {
-								const ext = preNode.firstChild.className.match(/language-(\S+)/)?.[1] || "txt"
+								let ext = preNode.firstChild.className.match(/language-(\S+)/)?.[1]
+								if (!dUtils.supportedPlaintextPreviewExtensions.has(ext)) ext = "txt"
 								const filename = `inline_code.${ext}`
 								// Build the replacement <code> node
 								const replacementCode = doc.createElement("code")

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

@@ -1155,6 +1155,38 @@ test("event2message: code blocks are uploaded as attachments instead if they con
 	)
 })
 
+test("event2message: code blocks are uploaded as attachments instead if they contain incompatible backticks (use txt extension if discord does not recognise the language)", async t => {
+	t.deepEqual(
+		await eventToMessage({
+			type: "m.room.message",
+			sender: "@cadence:cadence.moe",
+			content: {
+				msgtype: "m.text",
+				body: "wrong body",
+				format: "org.matrix.custom.html",
+				formatted_body: 'So if you run code like this<pre><code class="language-if">System.out.println("```");</code></pre>it should print a markdown formatted code block'
+			},
+			event_id: "$pGkWQuGVmrPNByrFELxhzI6MCBgJecr5I2J3z88Gc2s",
+			room_id: "!kLRqKKUQXcibIMtOpl:cadence.moe"
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "cadence [they]",
+				content: "So if you run code like this `[inline_code.txt]` it should print a markdown formatted code block",
+				attachments: [{id: "0", filename: "inline_code.txt"}],
+				pendingFiles: [{name: "inline_code.txt", buffer: Buffer.from('System.out.println("```");')}],
+				avatar_url: undefined,
+				allowed_mentions: {
+					parse: ["users", "roles"]
+				}
+			}]
+		}
+	)
+})
+
 test("event2message: code blocks are uploaded as attachments instead if they contain incompatible backticks (default to txt file extension)", async t => {
 	t.deepEqual(
 		await eventToMessage({