Then I went to the website and the edit post interface try to replace the .m4a with an .mp3.

When I tried to edit, the only sign of the .m4a file was a bit of text or code at the bottom of the post body. 

As a first step, I deleted that. 

But I couldn't see any obvious way to add the .mp3 file (and the little player in which posterous so kindly bundles it up), so I saved the edit and went back to the blog to see if I'd actually deleted the m4a file.

I had, but I figured there had to be some way to attach an .mp3, so I tried editing again. 

But, alas, I think I might have broken something.  I can't edit the body of a bunch of my posts anymore.  Some, mysteriously, I can, but definitely not all.

Anyway, I don't know what the problem is, and I don't know if the posterous folks troll these posts for functionality feedback or vague bug reports, but I figure since posterous gives me this tool for free, I should throw this little experience up here just in case it might be of some use.

And, of course, I need to get the song up somehow.