Change stat to use Filepicker's metadata call instead of HEAD to allow r...

...etrieval of more information

The original 'stat' call used a HEAD request against a file's URL to retrieve some basic information about the file. Filepicker supports a GET request to a file's "/metadata" path to retrieve the same information in addition to some useful additional fields, such as width and height for image files.

Breaking Changes:
* "mime_type" is now named "mimetype" everywhere it is used to match Filepicker's "metadata" output.