Error code 400 while downloading a file via REST API

As shown above, >> (double redirect) should append the data (HTTP response) to the file. If you are using curl, -o (hyphen o) option is also handy.

Regarding the file extension, it depends on the type of file (data) you download. If you are downloading an jpeg image, then its' natural to add the .jpg extension to the filename and open with an image viewer (although Unix does not rely upon file extensions, I personally prefer to add appropriate one). If the data is text, yes, use .txt and open with an text editor of your choice.