First slice of csv upload of phone numbers for sending messages.

At the moment the file contents are not persisted by checked in
memory.

The first and last three records are show if all are valid.

If there are invalid rows, they are reported and the user is
prompted to go back and sort out upload file.

The storing of upload result (i.e. validation of file) in session
will be removed in next story which is about persisting of file
for later processing.

app/main/validators.py

@@ -38,3 +38,13 @@ class ValidateUserCodes(object):
                     break
         if not valid_code:
             raise ValidationError(self.invalid_msg)
+
+
+class CsvFileValidator(object):
+
+    def __init__(self, message='Not a csv file'):
+        self.message = message
+
+    def __call__(self, form, field):
+        if not form.file.data.mimetype == 'text/csv':
+            raise ValidationError(self.message)