diff --git a/qutebrowser/browser/qutescheme.py b/qutebrowser/browser/qutescheme.py
index 41d2cabaf..dc952f0f8 100644
--- a/qutebrowser/browser/qutescheme.py
+++ b/qutebrowser/browser/qutescheme.py
@@ -29,7 +29,6 @@ import json
 import os
 import time
 import textwrap
-import mimetypes
 import urllib
 import collections
 import base64
@@ -368,8 +367,7 @@ def qute_help(url):
             bdata = utils.read_file(path, binary=True)
         except OSError as e:
             raise SchemeOSError(e)
-        mimetype, _encoding = mimetypes.guess_type(urlpath)
-        assert mimetype is not None, url
+        mimetype = utils.guess_mimetype(urlpath)
         return mimetype, bdata
 
     try:
@@ -527,9 +525,7 @@ def qute_pdfjs(url):
         filename = QUrlQuery(url).queryItemValue('filename')
         with open(filename, 'rb') as f:
             data = f.read()
-        mimetype, _encoding = mimetypes.guess_type(filename)
-        if mimetype is None:
-            mimetype = 'application/octet-stream'
+        mimetype = utils.guess_mimetype(filename, fallback=True)
         return mimetype, data
 
     if url.path() == '/web/viewer.html':
@@ -547,7 +543,5 @@ def qute_pdfjs(url):
             "pdfjs resource requested but not found: {}".format(e.path))
         raise NotFoundError("Can't find pdfjs resource '{}'".format(e.path))
     else:
-        mimetype, _encoding = mimetypes.guess_type(url.fileName())
-        if mimetype is None:
-            mimetype = 'application/octet-stream'
+        mimetype = utils.guess_mimetype(url.fileName(), fallback=True)
         return mimetype, data
diff --git a/qutebrowser/utils/jinja.py b/qutebrowser/utils/jinja.py
index b06444f93..d2ddfaeb7 100644
--- a/qutebrowser/utils/jinja.py
+++ b/qutebrowser/utils/jinja.py
@@ -22,7 +22,6 @@
 import os
 import os.path
 import contextlib
-import mimetypes
 import html
 
 import jinja2
@@ -108,9 +107,8 @@ class Environment(jinja2.Environment):
         """Get a data: url for the broken qutebrowser logo."""
         data = utils.read_file(path, binary=True)
         filename = utils.resource_filename(path)
-        mimetype = mimetypes.guess_type(filename)
-        assert mimetype is not None, path
-        return urlutils.data_url(mimetype[0], data).toString()
+        mimetype = utils.guess_mimetype(filename)
+        return urlutils.data_url(mimetype, data).toString()
 
     def getattr(self, obj, attribute):
         """Override jinja's getattr() to be less clever.
diff --git a/qutebrowser/utils/utils.py b/qutebrowser/utils/utils.py
index fe255331d..e9d4c3659 100644
--- a/qutebrowser/utils/utils.py
+++ b/qutebrowser/utils/utils.py
@@ -33,6 +33,7 @@ import contextlib
 import socket
 import shlex
 import glob
+import mimetypes
 
 from PyQt5.QtCore import QUrl
 from PyQt5.QtGui import QColor, QClipboard, QDesktopServices
@@ -683,3 +684,19 @@ def chunk(elems, n):
         raise ValueError("n needs to be at least 1!")
     for i in range(0, len(elems), n):
         yield elems[i:i + n]
+
+
+def guess_mimetype(filename, fallback=False):
+    """Guess a mimetype based on a filename.
+
+    Args:
+        filename: The filename to check.
+        fallback: Fall back to application/octet-stream if unknown.
+    """
+    mimetype, _encoding = mimetypes.guess_type(filename)
+    if mimetype is None:
+        if fallback:
+            return 'application/octet-stream'
+        else:
+            raise ValueError("Got None mimetype for {}".format(filename))
+    return mimetype
diff --git a/tests/unit/utils/test_utils.py b/tests/unit/utils/test_utils.py
index 550213386..e8a96d448 100644
--- a/tests/unit/utils/test_utils.py
+++ b/tests/unit/utils/test_utils.py
@@ -816,3 +816,16 @@ def test_chunk(elems, n, expected):
 def test_chunk_invalid(n):
     with pytest.raises(ValueError):
         list(utils.chunk([], n))
+
+
+@pytest.mark.parametrize('filename, expected', [
+    ('test.jpg', 'image/jpeg'),
+    ('test.blabla', 'application/octet-stream'),
+])
+def test_guess_mimetype(filename, expected):
+    assert utils.guess_mimetype(filename, fallback=True) == expected
+
+
+def test_guess_mimetype_no_fallback():
+    with pytest.raises(ValueError):
+        utils.guess_mimetype('test.blabla')