Fix bug with large file uploads

Depending on the size of the uploaded file, Flask will temporarily store
it in different ways. This means that it comes back as a `TempFile` if
the file is roughly <500k and as `BytesIO` if the file is larger.

`TempFile` supports the `.getvalue()` method, but `BytesIO` does not.
Both support the `.read()` method, so this commit changes to use that
instead.

app/main/views/send.py

@@ -110,7 +110,7 @@ def send_messages(service_id, template_id):
                 service_id,
                 {
                     'file_name': form.file.data.filename,
-                    'data': form.file.data.getvalue().decode('utf-8')
+                    'data': form.file.data.read().decode('utf-8')
                 },
                 current_app.config['AWS_REGION']
             )