Attachment bug fix (#3818)

* Prevent name check on null attachment file

(cherry picked from commit c4ed1e23a01f278d696c2853337bdde0a682c6c5)

* Unit testing for uploading attachments via API

(cherry picked from commit 592548065f7b69f58b8aaaaea506e3ec653a63df)

InvenTree/part/test_api.py

@@ -2350,3 +2350,83 @@ class PartParameterTest(InvenTreeAPITestCase):
         data = response.data
 
         self.assertEqual(data['data'], '15')
+
+
+class PartAttachmentTest(InvenTreeAPITestCase):
+    """Unit tests for the PartAttachment API endpoint"""
+
+    fixtures = [
+        'category',
+        'part',
+        'location',
+    ]
+
+    def test_add_attachment(self):
+        """Test that we can create a new PartAttachment via the API"""
+
+        url = reverse('api-part-attachment-list')
+
+        # Upload without permission
+        response = self.post(
+            url,
+            {},
+            expected_code=403,
+        )
+
+        # Add required permission
+        self.assignRole('part.add')
+
+        # Upload without specifying part (will fail)
+        response = self.post(
+            url,
+            {
+                'comment': 'Hello world',
+            },
+            expected_code=400
+        )
+
+        self.assertIn('This field is required', str(response.data['part']))
+
+        # Upload without file OR link (will fail)
+        response = self.post(
+            url,
+            {
+                'part': 1,
+                'comment': 'Hello world',
+            },
+            expected_code=400
+        )
+
+        self.assertIn('Missing file', str(response.data['attachment']))
+        self.assertIn('Missing external link', str(response.data['link']))
+
+        # Upload an invalid link (will fail)
+        response = self.post(
+            url,
+            {
+                'part': 1,
+                'link': 'not-a-link.py',
+            },
+            expected_code=400
+        )
+
+        self.assertIn('Enter a valid URL', str(response.data['link']))
+
+        link = 'https://www.google.com/test'
+
+        # Upload a valid link (will pass)
+        response = self.post(
+            url,
+            {
+                'part': 1,
+                'link': link,
+                'comment': 'Hello world',
+            },
+            expected_code=201
+        )
+
+        data = response.data
+
+        self.assertEqual(data['part'], 1)
+        self.assertEqual(data['link'], link)
+        self.assertEqual(data['comment'], 'Hello world')