Fix #4546 (#4548)

* Refactor download

* emoji type

src/misc/detect-mine.ts

@@ -0,0 +1,31 @@
+import * as fs from 'fs';
+import fileType from 'file-type';
+import checkSvg from '../misc/check-svg';
+
+export async function detectMine(path: string) {
+	return new Promise<[string, string]>((res, rej) => {
+		const readable = fs.createReadStream(path);
+		readable
+			.on('error', rej)
+			.once('data', (buffer: Buffer) => {
+				readable.destroy();
+				const type = fileType(buffer);
+				if (type) {
+					if (type.mime == 'application/xml' && checkSvg(path)) {
+						res(['image/svg+xml', 'svg']);
+					} else {
+						res([type.mime, type.ext]);
+					}
+				} else if (checkSvg(path)) {
+					res(['image/svg+xml', 'svg']);
+				} else {
+					// 種類が同定できなかったら application/octet-stream にする
+					res(['application/octet-stream', null]);
+				}
+			})
+			.on('end', () => {
+				// maybe 0 bytes
+				res(['application/octet-stream', null]);
+			});
+	});
+}