[Validator/Helper] Add validator & helper method to facilitate microsoft documents to content_type validator

We could add a shorthand for msword, msexcel, msppt documents in our `content_type`  validator.
This shorthand could also be made available to the views to alleviate the effort of writing something like:
```erb
<%= form.file_field :document, accept: ".doc,.docx,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document" %>
```