Get mime type from getDocumentType instead of from file name

DownloadProvider and FileSystemProvide will decide mime type
in different way, so if some function like getDocumentMetadata
will try to get mime type and do some loading. It may reurn unsupported

Bug: 80454526
Test: Download jpg file, remove .jpg extension, get info
Change-Id: Ia8d237181eba51005981ba46a1be3a43af0f249c