Remove reference to 'index' in docs for fileFormDataName

Since #217, the fileFormDataName has no longer been appended with an index.

Also added a little more detail about how the user might want to add square brackets to the fileFormDataName, which seems to be required in my use case (Rails).
Merge pull request #268 from tjgrathwell/patch-1

Remove reference to 'index' in docs for fileFormDataName