diff --git a/src/com/annimon/ownlang/lib/modules/files.java b/src/com/annimon/ownlang/lib/modules/files.java
index 68b8c35..475a58a 100644
--- a/src/com/annimon/ownlang/lib/modules/files.java
+++ b/src/com/annimon/ownlang/lib/modules/files.java
@@ -94,10 +94,11 @@ public final class files implements Module {
             
             DataOutputStream dos = null;
             BufferedWriter writer = null;
+            final boolean append = mode.contains("+");
             if (mode.contains("wb")) {
-                dos = new DataOutputStream(new FileOutputStream(file));
+                dos = new DataOutputStream(new FileOutputStream(file, append));
             } else if (mode.contains("w")) {
-                writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file), "UTF-8"));
+                writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file, append), "UTF-8"));
             }
             
             final int key = files.size();
@@ -115,7 +116,7 @@ public final class files implements Module {
             try {
                 return execute(files.get(key), args);
             } catch (IOException ioe) {
-                return NumberValue.ZERO;
+                return NumberValue.MINUS_ONE;
             }
         }
         
@@ -168,8 +169,6 @@ public final class files implements Module {
         @Override
         protected Value execute(FileInfo fileInfo, Value[] args) throws IOException {
             final File dest = files.get(args[1].asInt()).file;
-            System.out.println(fileInfo.file);
-            System.out.println(dest);
             return NumberValue.fromBoolean(fileInfo.file.renameTo(dest));
         }
     }
@@ -367,7 +366,13 @@ public final class files implements Module {
     private static class writeLong extends FileFunction {
         @Override
         protected Value execute(FileInfo fileInfo, Value[] args) throws IOException {
-            fileInfo.dos.writeLong((long) args[1].asNumber());
+            final long value;
+            if (args[1].type() == Types.NUMBER) {
+                value = ((NumberValue)args[1]).asLong();
+            } else {
+                value = (long) args[1].asNumber();
+            }
+            fileInfo.dos.writeLong(value);
             return NumberValue.ONE;
         }
     }
@@ -375,7 +380,13 @@ public final class files implements Module {
     private static class writeFloat extends FileFunction {
         @Override
         protected Value execute(FileInfo fileInfo, Value[] args) throws IOException {
-            fileInfo.dos.writeFloat((float) args[1].asNumber());
+            final float value;
+            if (args[1].type() == Types.NUMBER) {
+                value = ((NumberValue)args[1]).asFloat();
+            } else {
+                value = (float) args[1].asNumber();
+            }
+            fileInfo.dos.writeFloat(value);
             return NumberValue.ONE;
         }
     }
diff --git a/tests.own b/tests.own
index 620ac4c..862f935 100644
--- a/tests.own
+++ b/tests.own
@@ -1,6 +1,8 @@
 use "ounit"
+use "types"
 use "functional"
 use "date"
+use "files"
 
 def testAdditionOnNumbers() {
   assertEquals(6, 0 + 1 + 2 + 3)
@@ -68,13 +70,6 @@ def testFunctionalChain() {
   assertEquals([8,6,4,2], result)
 }
 
-def testDateParse() {
-  d = parseDate("2016/05/10", newFormat("yyyy/MM/dd"))
-  assertEquals(2016, d.year)
-  assertEquals(4, d.month)
-  assertEquals(10, d.day)
-  assertEquals(0, d.hour)
-}
 
 // --- Date
 def testNewDate() {
@@ -107,4 +102,30 @@ def testDateParse() {
   assertEquals(0, d.hour)
 }
 
+// --- Files
+def testFiles() {
+  // writeLong
+  f = fopen("test.file", "wb")
+  writeLong(f, 1002003004005006007)
+  flush(f)
+  fclose(f)
+
+  // append & writeFloat
+  fpNumber = 100200.3004005006007
+  f = fopen("test.file", "wb+")
+  writeFloat(f, fpNumber)
+  flush(f)
+  fclose(f)
+
+  f = fopen("test.file", "rb")
+  assertEquals(1002003004005006007, readLong(f))
+  assertEquals(float(fpNumber), readFloat(f))
+  assertEquals(-1, readInt(f)) // EOF
+  fclose(f)
+
+  f = fopen("test.file", "i")
+  delete(f)
+  fclose(f)
+}
+
 println runTests()
\ No newline at end of file