Why do I sometimes get double extension names in OS X?

Question

Sometimes when downloading files, I end up with a file like "foo.txt.txt". Sometimes I end up with just "foo" (and then manually change the extension later).

Is this a common problem with OS X? What is the cause? Are there some simple principles to follow to avoid it happening? I assume it's something to do with the way OS X doesn't actually care about file extensions, whereas Windows does.

Answer 1

I've done this numerous times in other environments, but this typically happens when you name a file and whatever saving routine from within the application you are using automatically appends an extension to the end of a file, even if you keyed in the file extension when saving the file.

Furthermore, this may also happen if file extensions are hidden within Finder and you rename a file and include the extension.