Handle capitalized and lowercase file types during retrieval

I ran into a bug trying to download an Excel file, where the file_type variable was not being set. Looking at the `dryad_getfile()` function, it was searching the URL for `.XLS` so it was missing the lowercase `.xls` in data file I was attempting to downlaod. I've modified the first set of conditionals to ignore case when matching the file type. The function also pulls out the URL path and only searches at the end of that string (using `grepl()`).