Change the extension on the downloaded file to
.txt or use the option to "choose" the application to use when double-clicking to open the file (example: TextEdit). You can set up your preferences to always open files with the given plain text extension (tabular, bed, sam, gtf/gff, etc) with TextEdit by saving the file once open within that application. Be aware that very large data may not fully open or exceed memory - it depends on your computer's resources.
If you want to access it line-command (vi, emacs, or input to a line-command tool), the extension usually does not matter. All plain text files are treated the same, only binary/compressed files would be unreadable (uncompress as needed).
Hope that helps! Jen, Galaxy team