Get binary contents in Zotero.File.getSample()

And fix magic numbers for content-type sniffing, which wrongly used the
Unicode replacement character (which likely just meant we were falling
back to file-extension-based detection)

test/tests/mimeTest.js

@@ -0,0 +1,22 @@
+describe("Zotero.MIME", function () {
+	describe("#sniffForMIMEType()", function () {
+		async function test(filename, expectedType) {
+			var path = OS.Path.join(getTestDataDirectory().path, filename);
+			var sample = await Zotero.File.getSample(path);
+			var type = Zotero.MIME.sniffForMIMEType(sample);
+			assert.equal(type, expectedType);
+		}
+		
+		it("should detect PNG", async function () {
+			await test('test.png', 'image/png');
+		});
+		
+		it("should detect JPEG", async function () {
+			await test('test.jpg', 'image/jpeg');
+		});
+		
+		it("should detect SQLite database", async function () {
+			await test('dev@zotero.org@www.mendeley.com.sqlite', 'application/x-sqlite3');
+		});
+	});
+});