See #5124.

The Haskell function is in module Text.Pandoc.SelfContained, but it seems to fit into the pandoc.mediabag Lua module conceptually.

Ideas for the interface:

• pandoc.mediabag.make_data_uri(input[, mime]), with mime defaulting to application/octet-stream, or
• pandoc.mediabag.make_data_uri(input_or_filepath[, mime]) with pandoc trying to treat the input_or_filepath argument as the path to an item in the mediabag if mime is not given.