Composer: allow replacing a media attachment without losing alt text and ordering

[Alex0007]

Problem I have

In the composer, the only per-attachment actions are editing the description and removing the attachment. New files are always appended at the end, and once the attachment limit is reached (e.g. 4/4), adding files is blocked entirely with "You can only attach up to 4 files."

This makes replacing a single attachment painful. Real scenario I just hit: I attached 4 images and wrote alt text for each, then the server rejected the first image with a 422 (file size). To swap in a fixed version of that image in the same position, I had to remove all attachments and re-add them in order — losing all 4 alt texts I'd written.

The failing-upload case makes this worse: the rejected attachment just sits there with an error; there's no way to retry or point it at a fixed file.

Solution I'd like

Either one of these would solve the issue on its own (both would be even better):

• Replace action on an attachment — pick a new file for an existing slot, keeping its description and position. This would also be the natural retry path for failed uploads.
• Drag'n'drop reordering — then I could remove just the bad attachment, add the fixed file (it lands at the end), and drag it back into place, with the other attachments' alt texts untouched.

Alternatives considered

• Current workaround: remove all attachments and re-add them in the right order, re-typing every alt text. Works, but loses a lot of writing.
• Avoiding oversized files up front: doesn't fully help, because the server can reject files that are under the advertised limit, and no client-side check can predict that. My rejected file was a 13.0 MB PNG on mastodon.social (advertised image_size_limit is 16 MB) — Phanpy correctly showed no warning, yet the server returned 422 Validation failed: File must be less than 16 MB. The official web UI rejects the same file, so it's a server-side issue (possibly related to <16MB indexed color images detected as too big mastodon/mastodon#32983). Such rejections of valid-looking files will keep happening, so the composer needs a graceful recovery path.

Other

Where this lives in the code:

• Attachment items only get onDescriptionChange and onRemove: src/components/compose.jsx (MediaAttachment usage, ~line 1596).
• New files are appended: setMediaAttachments((prev) => [...prev, ...mediaFiles]).
• Hard block at max attachments: processFiles in src/components/compose.jsx (~line 290).