Added makeDownload() method to modFileHandler class

This comes handy when a file should be sent to the user as a download instead of saving it into the filesystem.

Also added new helper function getContent() (not the same as the existing getContents()) to get the temporarily set content for a fileobject, this is needed for the makeDownload() method when no file should be created in the filesystem, e.g. the content is only set via setContent() but never saved/created.

The last change is a small addition to the getSize() method to calculate the size of the content when it's not a file yet and filesize() can't work properly.