Work to improve mime type detection in attachment view

Work to improve mime type detection in attachment view
(fixes NB#57213, NB#57671, NB#57575).
* src/modest-platform.h:
        * (modest_platform_activate_file): now it gets a mime_type parameter,
          in case it was detected previously.
* src/gnome/modest-platform.c:
        * (modest_platform_activate_file): updated API to get mime type
* src/maemo/modest-platform.c:
        * (guess_mime_type_from_file): fixed a trivial bug, that broke the
          detection.
        * (modest_platform_activate_file): use mime type to open the file
          if available, or uses guess_mime_type if no type is provided.
* src/maemo/modest-msg-view-window.c:
        * (..._view_attachment). Preserve the extension on saving the
          attachments to temporary file to view it.
        * (..._view_attachment). Get the mime type of the mime part, and
          pass it to the activate_file call.
* src/maemo/modest-maemo-utils.c:
        * (..._create_temp_stream): now accepts a preferred extension for
          the temporary file.

pmo-trunk-r2067