The Tumblr API is returning spurious errors in response to attempts to upload photo-type posts. I've sent a support request but if anybody knows somebody who works on the Tumblr API at Automattic, please encourage them to get in touch! Thanks.

@danielpunkass I forwarded your toot and having someone looking into it 👍

(they didn't find any ticket from you at first, but it seems this was because you used random email addresses without your name when you submitted the ticket ; they're now confident they found the right ticket though)

@aligatr Thank you so much! Yeah, I don't think the form had a lot of opportunity to specify name or anything else.
@danielpunkass no worries, the ticket they found had references to marstesting site and MarsEdit user agent in the API calls so we figured we found the right one after all 😜
@aligatr Haha good call! I'm glad to know somebody's giving it a thought, at least! Thanks again.