diff --git a/file/sniffer.js b/file/sniffer.js
new file mode 100644
index 0000000..4696850
--- /dev/null
+++ b/file/sniffer.js
@@ -0,0 +1,63 @@
+/**
+ * File Sniffer.
+ *
+ * Licensed under the MIT License
+ *
+ * Copyright(c) 2020 Google Inc.
+ */
+
+const fileSignatures = {
+ 'image/png': [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A],
+};
+
+// TODO: Build a tree of signature bytes that end in a MIME type.
+class Node {
+ /**
+ * @param {number} value
+ */
+ constructor(value) {
+ this.value = value;
+ this.children = {};
+ this.mimeType = undefined;
+ }
+}
+
+// Top-level node in the tree.
+const root = new Node();
+let maxDepth = 0;
+
+// Construct the tree.
+(function() {
+ for (const mimeType in fileSignatures) {
+ let curNode = root;
+ let depth = 0;
+ for (const byte of fileSignatures[mimeType]) {
+ if (curNode.children[byte] === undefined) {
+ curNode.children[byte] = new Node(byte);
+ }
+ depth++;
+ curNode = curNode.children[byte];
+ }
+ if (maxDepth < depth) {
+ maxDepth = depth;
+ }
+ curNode.mimeType = mimeType;
+ }
+})();
+
+/**
+ * @param {ArrayBuffer} ab
+ * @return {string} The MIME type of the buffer, or undefined.
+ */
+export function findMimeType(ab) {
+ const depth = ab.byteLength < maxDepth ? ab.byteLength : maxDepth;
+ const arr = new Uint8Array(ab).subarray(0, depth);
+ let curNode = root;
+ for (const byte of arr) {
+ if (curNode.children[byte] === undefined) return undefined;
+ curNode = curNode.children[byte];
+ if (curNode.mimeType) return curNode.mimeType;
+ }
+}
+
+debugger;
\ No newline at end of file
diff --git a/tests/file-sniffer-test.html b/tests/file-sniffer-test.html
new file mode 100644
index 0000000..851f693
--- /dev/null
+++ b/tests/file-sniffer-test.html
@@ -0,0 +1,27 @@
+
+
+
+ Unit tests for bitjs.io.ByteStreamBitStream
+
+
+
+
\ No newline at end of file