add attachment model and task order relation to it

tests/models/test_attachment.py

@@ -0,0 +1,18 @@
+import pytest
+from werkzeug.datastructures import FileStorage
+
+from atst.models.attachment import Attachment, AttachmentError
+
+from tests.mocks import PDF_FILENAME
+
+
+def test_attach(pdf_upload):
+    attachment = Attachment.attach(pdf_upload)
+    assert attachment.filename == PDF_FILENAME
+
+
+def test_attach_raises():
+    with open(PDF_FILENAME, "rb") as fp:
+        fs = FileStorage(fp, content_type="something/else")
+        with pytest.raises(AttachmentError):
+            Attachment.attach(fs)