That bug has been described on https://launchpad.net/distros/ubuntu/+source/serpentine/+bug/31601

"I'm running Serpentine v0.6.9.0 in Dapper Drake (testing). When I attempt to save a playlist, Serpentine has no playlist format selected by default. Instead it has "Detect by extension" selected. This is extremely technical for the average user. If one tries to create a playlist called "CD Project 1", an error message occurs with the following "Unsupported Format: The playlist format you used (by the file extension) is currently not supported."

Serpentine should automatically do "the right thing" when a user tries to save a playlist. There should be no extension required from the user and I am inclined to think that there should not even be a playlist format option in the save dialog as it overly complicates the playlist saving process. At the very least, Serpentine should automatically select a file format when the user does not provide a file extension in the filename.

As far as which file format should be used by default, I am unsure which to propose. Two requisites I can think of for the default file format are that Serpentine should be capable of opening that format and the format should be an open, non-patent-encumbered standard. Interoperability is another factor to consider when selecting a default format for Serpentine to use. If a user creates several playlists with Serpentine then he or she may eventually wish to open those playlists with Totem or Rhythmbox as well."