We should have helper methods / extensions for DataContent

[ericstj]

At a minimum we should have Load and Save.

Load should read from a path and preserve the file name loaded and try to decode a mime-type from that.

Save should save to a path, with an optional filename, using Name if a directory is provided in the path.

Perhaps we could consider a filename without extension to be provided and we infer the extension from the mime-type and return the saved path.

We should also have Stream based helpers.