diff --git a/types/file-type/file-type-tests.ts b/types/file-type/file-type-tests.ts
index 640d2c8fb0..d49a964829 100644
--- a/types/file-type/file-type-tests.ts
+++ b/types/file-type/file-type-tests.ts
@@ -1,6 +1,8 @@
 import fileType = require("file-type");
 
+// $ExpectType FileTypeResult | null
 fileType(new Buffer([0xFF, 0xD8, 0xFF]));
 fileType(new Uint8Array([0xFF, 0xD8, 0xFF]));
 
-const minimumBytes: number = fileType.minimumBytes;
+// $ExpectType number
+fileType.minimumBytes;
diff --git a/types/file-type/index.d.ts b/types/file-type/index.d.ts
index 2a0deee26a..c902dc384e 100644
--- a/types/file-type/index.d.ts
+++ b/types/file-type/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for file-type 5.2
+// Type definitions for file-type 10.6
 // Project: https://github.com/sindresorhus/file-type
 // Definitions by: KIM Jaesuck a.k.a. gim tcaesvk <https://github.com/tcaesvk>
 //                 BendingBender <https://github.com/BendingBender>
@@ -8,7 +8,17 @@
 
 export = FileType;
 
-declare function FileType(buf: Buffer | Uint8Array): FileType.FileTypeResult;
+/**
+ * The file type is detected by checking the
+ * [magic number](http://en.wikipedia.org/wiki/Magic_number_(programming)#Magic_numbers_in_files) of the buffer.
+ *
+ * @param buf It only needs the first `.minimumBytes` bytes. The exception is detection of
+ *            `docx`, `pptx`, and `xlsx` which potentially requires reading the whole file.
+ * @returns an Object with:
+ * - ext - One of the [supported file types](https://github.com/sindresorhus/file-type/blob/master/readme.md#supported-file-types)
+ * - mime - The [MIME type](http://en.wikipedia.org/wiki/Internet_media_type)
+ */
+declare function FileType(buf: Buffer | Uint8Array): FileType.FileTypeResult | null;
 
 declare namespace FileType {
     interface FileTypeResult {
@@ -16,5 +26,9 @@ declare namespace FileType {
         mime: string;
     }
 
+    /**
+     * The minimum amount of bytes needed to detect a file type. Currently,
+     * it's 4100 bytes, but it can change, so don't hard-code it.
+     */
     const minimumBytes: number;
 }
diff --git a/types/file-type/tsconfig.json b/types/file-type/tsconfig.json
index 499b81459c..1894b908af 100644
--- a/types/file-type/tsconfig.json
+++ b/types/file-type/tsconfig.json
@@ -6,7 +6,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [
@@ -20,4 +20,4 @@
         "index.d.ts",
         "file-type-tests.ts"
     ]
-}
\ No newline at end of file
+}