the html content of the article has an img tag with a relative URL to "cat.jpg" and there is also a file in the request called "cat.jpg"

post.txt

POST /micropub HTTP/1.1
Content-type: multipart/form-data; boundary=1884ea44ff5ec7421c3becf56820d9e0
Authorization: Bearer xxxxxxxxxx

--1884ea44ff5ec7421c3becf56820d9e0
Content-Disposition: form-data; name="h"

entry
--1884ea44ff5ec7421c3becf56820d9e0
Content-Disposition: form-data; name="content[html]"

<h2>My Cat</h2><p><img src="cat.jpg"></p>
--1884ea44ff5ec7421c3becf56820d9e0
Content-Disposition: form-data; name="cat.jpg"; filename="/tmp/14987532"
Content-Type: image/jpg
Content-Transfer-Encoding: binary

(binary image data)
--1884ea44ff5ec7421c3becf56820d9e0--