diff --git a/Zip/Zip.swift b/Zip/Zip.swift
index e062b14267567dd51c8f8d1e08c1ca3ac4473627..1284835c7756fb6fa3b5f76fa6073488ce35c1f4 100644
--- a/Zip/Zip.swift
+++ b/Zip/Zip.swift
@@ -31,6 +31,11 @@ public enum ZipError: ErrorType {
 /// Zip class
 public class Zip {
     
+    /**
+     Set of vaild file extensions
+     */
+    internal static var customFileExtensions: Set<String> = []
+    
     // MARK: Lifecycle
     
     /**
@@ -308,13 +313,45 @@ public class Zip {
      
      - parameter fileExtension: A file extension.
      
-     - returns: false if the extension is "zip" or "cbz", otherwise true.
+     - returns: false if the extension is a valid file extension, otherwise true.
      */
     internal class func fileExtensionIsInvalid(fileExtension: String?) -> Bool {
         
         guard let fileExtension = fileExtension else { return true }
         
-        return !["zip", "cbz"].contains(fileExtension)
+        return !isValidFileExtension(fileExtension)
     }
     
-}
\ No newline at end of file
+    /**
+     Add a file extension to the set of custom file extensions
+     
+     - parameter fileExtension: A file extension.
+     */
+    public class func addCustomFileExtension(fileExtension: String) {
+        customFileExtensions.insert(fileExtension)
+    }
+    
+    /**
+     Remove a file extension from the set of custom file extensions
+     
+     - parameter fileExtension: A file extension.
+     */
+    public class func removeCustomFileExtension(fileExtension: String) {
+        customFileExtensions.remove(fileExtension)
+    }
+    
+    /**
+     Check if a specific file extension is valid
+     
+     - parameter fileExtension: A file extension.
+     
+     - returns: true if the extension valid, otherwise false.
+     */
+    public class func isValidFileExtension(fileExtension: String) -> Bool {
+        
+        let validFileExtensions: Set<String> = customFileExtensions.union(["zip", "cbz"])
+        
+        return validFileExtensions.contains(fileExtension)
+    }
+    
+}
diff --git a/ZipTests/ZipTests.swift b/ZipTests/ZipTests.swift
index 2b59732e041af05d219f3cdb439b93ba2bdb0cc6..3f3880fcaf3feee0131aceeacdb972c358a4af7b 100644
--- a/ZipTests/ZipTests.swift
+++ b/ZipTests/ZipTests.swift
@@ -209,7 +209,7 @@ class ZipTests: XCTestCase {
         }
     }
 
-    func testFileExtensionIsInvalidForValidUrl() {
+    func testFileExtensionIsNotInvalidForValidUrl() {
         let fileUrl = NSURL(string: "file.cbz")
         let result = Zip.fileExtensionIsInvalid(fileUrl?.pathExtension)
         XCTAssertFalse(result)
@@ -220,4 +220,33 @@ class ZipTests: XCTestCase {
         let result = Zip.fileExtensionIsInvalid(fileUrl?.pathExtension)
         XCTAssertTrue(result)
     }
+    
+    func testAddedCustomFileExtensionIsValid() {
+        let fileExtension = "cstm"
+        Zip.addCustomFileExtension(fileExtension)
+        let result = Zip.isValidFileExtension(fileExtension)
+        XCTAssertTrue(result)
+        Zip.removeCustomFileExtension(fileExtension)
+    }
+    
+    func testRemovedCustomFileExtensionIsInvalid() {
+        let fileExtension = "cstm"
+        Zip.addCustomFileExtension(fileExtension)
+        Zip.removeCustomFileExtension(fileExtension)
+        let result = Zip.isValidFileExtension(fileExtension)
+        XCTAssertFalse(result)
+    }
+    
+    func testDefaultFileExtensionsIsValid() {
+        XCTAssertTrue(Zip.isValidFileExtension("zip"))
+        XCTAssertTrue(Zip.isValidFileExtension("cbz"))
+    }
+    
+    func testDefaultFileExtensionsIsNotRemoved() {
+        Zip.removeCustomFileExtension("zip")
+        Zip.removeCustomFileExtension("cbz")
+        XCTAssertTrue(Zip.isValidFileExtension("zip"))
+        XCTAssertTrue(Zip.isValidFileExtension("cbz"))
+    }
+    
 }