diff --git a/flake.nix b/flake.nix
index 32e9322..3daa679 100644
--- a/flake.nix
+++ b/flake.nix
@@ -20,7 +20,10 @@
         version = "0.0.0";
         src = self;
         nativeBuildInputs = [ nixpkgs.makeWrapper ];
-        buildInputs = [ nixpkgs.python3 libgourou-utils inept-epub ];
+        buildInputs = [
+          (nixpkgs.python3.withPackages(python3Packages: [ python3Packages.python_magic ]))
+          libgourou-utils inept-epub
+        ];
         installPhase = ''
           mkdir -p $out/bin
           chmod +x knock
diff --git a/knock b/knock
index 231b194..6850541 100644
--- a/knock
+++ b/knock
@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-import os, sys, argparse, subprocess
+import os, sys, argparse, subprocess, magic
 from pathlib import Path
 from getpass import getpass
 
@@ -10,15 +10,15 @@ args = parser.parse_args()
 
 # these are all Path objects: https://docs.python.org/3/library/pathlib.html
 args.acsm_file = Path(args.acsm_file).expanduser()
-args.drm_file = args.acsm_file.with_suffix('.drm.epub')
+args.drm_file = args.acsm_file.with_suffix('.drm')
 args.epub_file = args.acsm_file.with_suffix('.epub')
 args.adobe_dir = Path('~/.config/knock').expanduser()
 
 if not args.acsm_file.exists():
-    raise Exception(f'{str(args.acsm_file)} does not exist.')
+    sys.exit(f'{str(args.acsm_file)} does not exist.')
 
 if args.epub_file.exists():
-    raise Exception(f'{str(args.epub_file)} already exists.')
+    sys.exit(f'{str(args.epub_file)} already exists.')
 
 if not args.adobe_dir.exists():
     print('This device is not registered with Adobe.')
@@ -44,6 +44,10 @@ subprocess.check_call([
     '-f', str(args.acsm_file)
 ])
 
+drm_file_type = magic.from_file(str(args.drm_file), mime=True)
+if drm_file_type != 'application/epub+zip':
+    sys.exit(f'Received a file of type:\n{drm_file_type}\nKnock only supports EPUB files.')
+
 print('Decrypting the file...')
 
 subprocess.check_call([