Use OS.File for file reads in Zotero.File.get(Binary)ContentsAsync()

This is the recommended approach (since NetUtil can still do some main-thread
I/O for files) and avoids warnings in the console.

For getContentsAsync(), also sends nsIURIs and string URIs to
Zotero.HTTP.request(), which should be used instead.

This makes getBinaryContentsAsync() much slower (due to the conversion from an
array of bytes to a binary string), but it's only used in tests. For one test
that compares two large files, use MD5 instead.

test/tests/fileTest.js

@@ -38,8 +38,43 @@ describe("Zotero.File", function () {
 			assert.lengthOf(contents, 3);
 			assert.equal(contents, "A\uFFFDB");
 		})
+		
+		it("should respect maxLength", function* () {
+			var contents = yield Zotero.File.getContentsAsync(
+				OS.Path.join(getTestDataDirectory().path, "test.txt"),
+				false,
+				6
+			);
+			assert.lengthOf(contents, 6);
+			assert.equal(contents, "Zotero");
+		});
 	})
 	
+	describe("#getBinaryContentsAsync()", function () {
+		var magicPNG = ["89", "50", "4e", "47", "0d", "0a", "1a", "0a"].map(x => parseInt(x, 16));
+		
+		it("should return a binary string", function* () {
+			var contents = yield Zotero.File.getBinaryContentsAsync(
+				OS.Path.join(getTestDataDirectory().path, "test.png")
+			);
+			assert.isAbove(contents.length, magicPNG.length);
+			for (let i = 0; i < magicPNG.length; i++) {
+				assert.equal(magicPNG[i], contents.charCodeAt(i));
+			}
+		});
+		
+		it("should respect maxLength", function* () {
+			var contents = yield Zotero.File.getBinaryContentsAsync(
+				OS.Path.join(getTestDataDirectory().path, "test.png"),
+				magicPNG.length
+			);
+			assert.lengthOf(contents, magicPNG.length)
+			for (let i = 0; i < contents.length; i++) {
+				assert.equal(magicPNG[i], contents.charCodeAt(i));
+			}
+		});
+	});
+	
 	describe("#copyDirectory()", function () {
 		it("should copy all files within a directory", function* () {
 			var tmpDir = Zotero.getTempDirectory().path;