Add optional ext keyword argument to set_cover()

Previously, set_cover() would rely on the Python standard module
`imghdr` to determine the type of the specified cover image file.
Unfortunately, imghdr does not support SVG, so set_cover() could
not handle that type (which is permitted by the EPUB spec).

Detecting SVG files programmatically is nontrivial, and is generally
considered to require parsing the file source.

Instead, simply provide an option for the client to specify the file
type with the new keyword argument.

mkepub/mkepub.py

@@ -106,9 +106,11 @@ def add_font(self, name, data):
         self.fonts.append(name)
         self._add_file(pathlib.Path('fonts') / name, data)
 
-    def set_cover(self, data):
+    def set_cover(self, data, ext=None):
         """Set the cover image to the given data."""
-        self._cover = 'cover.' + imghdr.what(None, h=data)
+        if not ext:
+            ext = imghdr.what(None, h=data)
+        self._cover = 'cover.' + ext
         self._add_file(pathlib.Path('covers') / self._cover, data)
         self._write('cover.xhtml', 'EPUB/cover.xhtml', cover=self._cover)
 

mkepub/tests/test_mkepub.py

@@ -6,6 +6,7 @@
 import mkepub
 import pathlib
 import pytest
+from unittest import mock
 
 ###############################################################################
 
@@ -104,3 +105,33 @@ def test_add_file():
     with open('mkepub/tests/cover.jpg', 'rb') as file:
         book._add_file('files/cover_1.jpg', file.read())
     assert (book.path / 'EPUB/files/cover_1.jpg').exists()
+
+
+@mock.patch("mkepub.Book._add_file", mock.MagicMock())
+@mock.patch("mkepub.Book._write", mock.MagicMock())
+class TestSetCover:
+    def test_set_cover_auto(self):
+        book = mkepub.Book('SetCover')
+
+        with open('mkepub/tests/cover.jpg', 'rb') as file:
+            data = file.read()
+        book.set_cover(data)
+        mkepub.Book._add_file.assert_called_with(pathlib.PosixPath('covers/cover.jpeg'), data)
+        mkepub.Book._write.assert_called_with(mock.ANY, mock.ANY, cover='cover.jpeg')
+
+    def test_set_cover_override(self):
+        book = mkepub.Book('SetCover')
+
+        with open('mkepub/tests/cover.jpg', 'rb') as file:
+            data = file.read()
+        book.set_cover(data, ext='png')
+        mkepub.Book._add_file.assert_called_with(pathlib.PosixPath('covers/cover.png'), data)
+        mkepub.Book._write.assert_called_with(mock.ANY, mock.ANY, cover='cover.png')
+
+    def test_set_cover_undetectable(self):
+        book = mkepub.Book('SetCover')
+
+        data = b'<svg></svg>'
+        book.set_cover(data, ext='svg')
+        mkepub.Book._add_file.assert_called_with(pathlib.PosixPath('covers/cover.svg'), data)
+        mkepub.Book._write.assert_called_with(mock.ANY, mock.ANY, cover='cover.svg')