matrix: send mimetype for image upload
matrix: send mimetype for image upload

Former-commit-id: f7cbb69083e80545ae828f38c51fa9be0d111603
Former-commit-id: 2f4e8cb18149186cbf232038ad8fc1da7f155afd
haste/ivar2,torhve/ivar2,torhve/ivar2,torhve/ivar2