add unittest for get_thumbnail

src/paperless_mail/tests/samples/simple_text.eml

@@ -0,0 +1,23 @@
+Return-Path: <mail@someserver.de>
+Delivered-To: mail@someserver.de
+Received: from mail.someserver.org ([::1])
+	by e1acdba3bd07 with LMTP
+	id KBKZGD2YR2NTCgQAjubtDA
+	(envelope-from <mail@someserver.de>)
+	for <mail@someserver.de>; Wed, 10 Oct 2022 11:40:46 +0200
+Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id 2BC9064C1616
+	for <some@one.de>; Wed, 12 Oct 2022 21:40:46 +0200 (CEST)
+Message-ID: <6e99e34d-e20a-80c4-ea61-d8234b612be9@someserver.de>
+Date: Wed, 12 Oct 2022 21:40:43 +0200
+MIME-Version: 1.0
+User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
+ Thunderbird/102.3.1
+Content-Language: en-US
+To: some@one.de
+From: Some One <mail@someserver.de>
+Content-Type: text/plain; charset=UTF-8; format=flowed
+Content-Transfer-Encoding: 7bit
+X-Last-TLS-Session-Version: TLSv1.3
+Subject: Simple Text Mail
+
+This is just a simple Text Mail.

src/paperless_mail/tests/test_eml.py

@@ -0,0 +1,39 @@
+import hashlib
+import os
+
+from django.test import TestCase
+from paperless_mail.parsers import MailDocumentParser
+
+
+class TestParser(TestCase):
+    SAMPLE_FILES = os.path.join(os.path.dirname(__file__), "samples")
+
+    def test_thumbnail(self):
+        def hashfile(file):
+            buf_size = 65536  # An arbitrary (but fixed) buffer
+            sha256 = hashlib.sha256()
+            with open(file, "rb") as f:
+                while True:
+                    data = f.read(buf_size)
+                    if not data:
+                        break
+                    sha256.update(data)
+            return sha256.hexdigest()
+
+        parser = MailDocumentParser(None)
+        thumb = parser.get_thumbnail(
+            os.path.join(self.SAMPLE_FILES, "simple_text.eml"),
+            "message/rfc822",
+        )
+        self.assertTrue(os.path.isfile(thumb))
+        thumb_hash = hashfile(thumb)
+
+        # The created intermediary pdf is not reproducible. But the thumbnail image should always look the same.
+        expected_hash = (
+            "18a2513c80584e538c4a129e8a2b0ce19bf0276eab9c95b72fa93e941db38d12"
+        )
+        self.assertEqual(
+            thumb_hash,
+            expected_hash,
+            "Thumbnail file hash not as expected.",
+        )