Restrict file upload to some file extensions

The answer to this question is probably more related to html uploading than rails.

When you want to upload a file, you typically do an input with type="file".

This can be done in Rails by using the file_field_tag helper. It will generate an input with type="file" which can also have an accept attribute, but you can't really use that because it's not really going to have any visible effect. This attribute accepts MIME types, not extensions, and most browsers don't even use it.

The best thing you can do is probably have a javascript check the file extension before upload (after you select the file from the dialog box). Read more about it in this question.

The point is, you can't force the OS to show you only the file extensions that you want. You can either validate the extension by using JS for example, before upload, or check the contents of the file after upload, server side

With HTML5 you can use the :accept for limiting mime-types, like so:

 <%= file_field_tag :csv_file,  :accept => 'text/csv' %>