Glad you got it sorted, thanks for letting us know about this!
Just had a couple of minutes to look at it, PR waiting.
Hmm... it looks like the endpoint already attempts to derive a mime type if the file upload doesn't provide one...
Do you have a capture of what micro.blog sends? I've signed up for an account, but not had time to really get into it and it's not immediately obvious how to send a post to my blog.
I'm wondering if it's a square bracket name issue.
Is there a capture of what micro.blog is actually sending?
Fix awaiting review, thanks for the spot!
Speculative fix awaiting review