Lua: make fetch return mime type first and then content.

doc/lua-filters.md

@@ -1135,6 +1135,10 @@ storage. The "media bag" is used when pandoc is called with the
 :   Returns a filename with a basename based on the SHA1 has of the
     contents and an extension based on the mime type.
 
+    Returns:
+
+    -   Filename based on SHA1 hash.
+
     Usage:
 
         local fp = pandoc.mediabag.hashname("plain/text", "foobar")
@@ -1145,6 +1149,11 @@ storage. The "media bag" is used when pandoc is called with the
     Returns two values:  the contents of the file and the mime
     type (or an empty string).
 
+    Returns:
+
+    -   the entries MIME type, or nil if the file was not found.
+    -   contents of the file, or nil if the file was not found.
+
     Usage:
 
         local diagram_url = "https://pandoc.org/diagram.jpg"

src/Text/Pandoc/Lua/PandocModule.hs

@@ -163,8 +163,8 @@ fetch commonState mbRef src = do
     putCommonState commonState
     setMediaBag mediaBag
     fetchItem src
-  Lua.push bs
   Lua.push $ fromMaybe "" mimeType
+  Lua.push bs
   return 2 -- returns 2 values: contents, mimetype
 
 --