We are using two libraries to do the same job. This commit removes the usage of mini_mime in favour of Marcel instead.

Changes are as follows:
- Replace MiniMime lookup by extension with Marcel Mimetype for lookup with extension
- Replaces usage of MiniMime lookup by content type to fetch extension with usage of Marcel Magic lookup. Marcel has multiple extentions being returned, we pick the first one. MiniMime always returns just one
- Removes specs which we specifically checking failing identification issue of MiniMine on jpeg images
- Removes mini_mime from gemspec