Limit length of filename

S3 has a limit of 2kb for metadata:

> the user-defined metadata is limited to 2 KB in size. The size of
> user-defined metadata is measured by taking the sum of the number of
> bytes in the UTF-8 encoding of each key and value.

– https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html#object-metadata

This means we have a limit of 1870 bytes for the filename:
```python
encoded = 'notification_count50000template_id665d26e7-ceac-4cc5-82ed-63d773d21561validTrueoriginal_file_name'.encode('utf-8')
sys.getsizeof(b)
>>> 130
2000-130
>>> 1870
```

Or, in other words, ~918 characters:
```python
sys.getsizeof(('ü'*918).encode('utf-8'))
>>> 1869
```

app/main/views/send.py

@@ -46,6 +46,7 @@ from app.utils import (
     get_errors_for_csv,
     get_help_argument,
     get_template,
+    unicode_truncate,
     user_has_permissions,
 )
 
@@ -555,7 +556,10 @@ def _check_messages(service_id, template_id, upload_id, preview_row, letters_as_
             notification_count=len(recipients),
             template_id=str(template_id),
             valid=True,
-            original_file_name=request.args.get('original_file_name'),
+            original_file_name=unicode_truncate(
+                request.args.get('original_file_name', ''),
+                1872
+            ),
         )
     else:
         session['file_uploads'].pop(upload_id)