posting text only fails from Pixelfed

Pixelfed doesn't support text only posts, every post has to have an image attached, so when I try it fails with this error. The posting interface should probably give some hint that you need to start with an image attachment

or you could have Phanpy post text but convert it to an image first.

**Version**
https://dev.phanpy.social build 2024.04.17.b9058c6
Mastodon account: https://social.wake.st/@liaizon
macOS 13.6.4
Firefox 124.0.2 (64-bit)

Start by reproducing a text-only post from the Phanpy posting interface against Pixelfed, using the reported build and Firefox setup as reference. Trace the posting flow to identify where the failure is surfaced and determine whether the intended result is an interface hint or conversion to an image. Done means the chosen behavior is implemented and the Pixelfed text-only case no longer fails without guidance.

Written by the indexing model from the issue text.