Support mime types fallback when loading remote files.

Added mime types lookup for files when loader tries to check for a file format. 

Also created a `getExtension` method in `X.file` instead of constructing it in `checkFileFormat`.

Custom mime types were used for file formats that didn't have anything official.

Took the empty string out of the extensions lookup and defaulted to 'application/dicom' if the lookup returns undefined. Ran all of the tests. It looks like selenium test for test_dcm.html was broken before these commits.