UnpackJob determines the MIME type of each file

Some destination document providers do not allow documents to be created
without a known non-empty MIME type.

Bug: 429498855
Flag: com.android.documentsui.flags.zip_ng_ro
Flag: com.android.documentsui.flags.use_material3
Test: atest  DocumentsUIGoogleTests:UnpackJobTest
Test: Manual with Google Drive
Change-Id: If3c56da4f3911798b1454bace7512caae172cc03