v3: simplify mimetypes

closes #136 (closed).

• remove usage of ctx.file.mime, always use libmagic's mimetype
• add ctx.resolve_mime() go have a tuple containing mime and extension to use
• make upload helper classes be dataclasses instead of namedtuples
• use 1kB instead of 512 bytes for libmagic handover
• add FORCE_EXTENSION so all files of a given mime go to a given extension (all jpeg images are .jpg)
• add INCLUDE_EXTENSION so we can add our own extensions to specific mimes (any application/octet-stream files can have an empty extension)
• ensure file stream is coherent when calculating hashes
• remove some app parameters