Add type restrictions to mime

[Vici37]

This is the output of compiling cr-source-typer and running it with the below incantation:

CRYSTAL_PATH="./src" ./typify spec/std_spec.cr
--error-trace --exclude src/crystal/ 
--stats --progress 
--ignore-private-defs 
--ignore-protected-defs  
src/mime

This is related to #15682 .