Since you're on Gmail, the most likely problem is (as documented in the information about getting a full dump of a message, headers and all, at the bottom of the page) that you copied-and-pasted the message with a blank line above the headers, the way Gmail presents it when you view the message source. The MIME-parsing library I use follows the standards and regards the headers as ending with the first blank line, meaning that it treats the headers as part of the body instead. Delete the blank line from the top of the message in the input box and it should work.
That fixed it - thanks!