Add new class MimeType
MimeType is defined now as: MimeType = str
. It's reasonable to create a new class MimeType where e.g. some of typical types are presented as class variables. A new class would also be usefull for type checking: now it's allowed to set any param to str where MimeType is required and it will not be detected. In a class it will also be possible to check if structure of MimeType value is correct.