Add check that PDF file can be opened and is not malformed

This checks that the PDF file is not malformed in some way (e.g. by
missing the EOF marker). We check this by trying to get the page count
of the letter which will be needed to display the preview of the letter.

tests/app/main/views/test_uploads.py

@@ -90,6 +90,20 @@ def test_post_choose_upload_file_when_file_is_too_big(mocker, client_request):
     assert normalize_spaces(page.select('.banner-dangerous')[0].text) == 'Your file must be smaller than 2MB'
 
 
+def test_post_choose_upload_file_when_file_is_malformed(mocker, client_request):
+    mocker.patch('app.main.views.uploads.antivirus_client.scan', return_value=True)
+
+    with open('tests/test_pdf_files/no_eof_marker.pdf', 'rb') as file:
+        page = client_request.post(
+            'main.upload_letter',
+            service_id=SERVICE_ONE_ID,
+            _data={'file': file},
+            _expected_status=400
+        )
+    assert page.find('h1').text == 'Upload a letter'
+    assert normalize_spaces(page.select('.banner-dangerous')[0].text) == 'Your file must be a valid PDF'
+
+
 def test_uploaded_letter_preview(client_request):
     page = client_request.get(
         'main.uploaded_letter_preview',