Fix opening markdown files
In d2ae2c28 we introduced a tweak that broke opening markdown files.
what we really want is this:
- If the mimetype from the filename is text (or children) but looking at the data says it's not text, then try opening first opening with the data suggeted mimetype
- But if the data suggested mimetype says it's text, use the filename suggested mimetype since for example, the only way to really differentiate markdown from text is the filename, so trust that