I think its best to move this to http folder, since we're having libs for HTTP Request and HTTP Response.
Its better to change MIME function to use an array. It would be much faster than a lot of ifs. Extension in key, MIME type in value.
IsModifiedSince should be protected.
Same goes for calculate... , also remove underlines from this. http can be lowercase.
readFromFile doesnt have a good name. Its not what it does. Also shouldnt be public
A large portion of the code is duplicated, merge it.
I don't know if Feed is also a good name. If you come up with something smarter use that. If you make everything else protected, anything public is a good remaining name :D