Allows to configure directory and filename formats for documents stored in paperless

Default configuration is as before (incrementing numbers), but additional fields can be added at will

src/documents/migrations/0023_document_current_filename.py

@@ -0,0 +1,37 @@
+# Generated by Django 2.0.10 on 2019-04-26 18:57
+
+from django.db import migrations, models
+
+
+def set_filename(apps, schema_editor):
+    Document = apps.get_model("documents", "Document")
+    for doc in Document.objects.all():
+        file_name = "{:07}.{}".format(doc.pk, doc.file_type)
+        if doc.storage_type == "gpg":
+            file_name += ".gpg"
+
+        # Set filename
+        doc.filename = file_name
+
+        # Save document
+        doc.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0022_auto_20181007_1420'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='document',
+            name='filename',
+            field=models.FilePathField(default=None,
+                                       null=True,
+                                       editable=False,
+                                       help_text='Current filename in storage',
+                                       max_length=256),
+        ),
+        migrations.RunPython(set_filename)
+    ]