The one concern when downloading a file is the file type. Text files are files which are solely composed of ascii text, for example a simpletext or html document. Almost all other files are binary. Word processing files, such as Microsoft Word files, may seem like they should be text but are actually binaries. This is because they have a lot of Microsoft specific information and code.
All programs are also binary.
Before you download a file you should specify which type the file is by using the three buttons at the bottom of the window. If you are unsure of the file type, choose the Automatic button. Fetch will make an educated guess as to what kind of file it is from its name, and treat it as a text file if it can't figure it out. If you are sure of the format of the file or if automatic is giving you problems, use the correct button for the file type. Most of the time, however, the Automatic setting will do the right thing.