There are three main modes for uploading files in an FTP client: auto, ASCII, and binary. All text-based files are uploaded in ASCII mode, whereas all images, movies, and audio files are uploaded using binary mode. Auto mode will automatically upload the file using the correct mode (ASCII or binary). Examples of ASCII file extensions include .html, .php, and .txt. Examples of binary files include MP3, JPG, and MOV.
Although auto mode automatically changes to ASCII or binary when necessary, it is not the most efficient method of uploading your files. Therefore, if you are planning to upload a batch of just text-based files or just binary files, you should use ASCII or binary, respectively. You can change the mode by navigating to Actions>Transfer Mode and selecting the proper mode. It's a good idea to switch the mode back to auto when you are done.