Explicitly spell out fallback element input type

[barbeque]

I didn't quite understand that this had to be a type="file" input element and I had to read the filedrop source code to figure it out.

If this is helpful, I could also create a pull request to throw an error on filedrop initialization if the fallback element is not of type file.

[barbeque]

I've gone ahead and made the following changes that I mentioned in the original post:

• Errors will no longer be raised when the fallback_id is unset
• If the fallback_id is set and the target element is not an input[type=file], it will raise an error: Fallback element [upload_button] must be of type 'file', but it is of 'button'.