emacs: Make the part content available to `mm-inlinable-p'.

emacs: Make the part content available to `mm-inlinable-p'.

The `mm-inlinable-p' function works better if it has access to the
data of the relevant part, so load that content before calling it.

Don't load the content for parts that the user has indicated no desire
to inline.

This fixes the display of attached image/jpeg parts, for example.