A picture alone is a comment on a video (#426)

ddiff --git a/server/comments/form.js b/server/comments/form.js
iindex 85d5f7a..305132d 100644
---- a/server/comments/form.js
++++ b/server/comments/form.js
@@@ -12,10 +12,11 @@ async function readCommentForm(req) {
      const request = new Request('http://localhost/comment', { method: 'POST', headers: { 'content-type': req.get('content-type') }, body: req.body });
      const form = await request.formData();
      const message = String(form.get('message') || '').trim();
--    if (!message || message.length > 2000) throw new Error('A comment is 1 to 2,000 characters');
++    const files = form.getAll('files').filter((f) => f && typeof f.arrayBuffer === 'function' && f.size);
++    // A picture alone is a comment, as on any social app (the owner, 11-12): words are optional with a file attached.
++    if ((!message && !files.length) || message.length > 2000) throw new Error('Write something or attach a picture (up to 2,000 characters)');
      const parent = String(form.get('parent_id') || '');
      if (parent && !ID.test(parent)) throw new Error('That reply target is not valid');
--    const files = form.getAll('files').filter((f) => f && typeof f.arrayBuffer === 'function' && f.size);
      if (files.length > 4) throw new Error('Attach up to four files');
      for (const file of files) {
          if (file.size > MAX_FILE) throw new Error('A file can be up to 16 MB');
ddiff --git a/test/comment-form.test.js b/test/comment-form.test.js
nnew file mode 100644
iindex 0000000..5ea2430
---- /dev/null
++++ b/test/comment-form.test.js
@@@ -0,0 +1,21 @@
++'use strict';
++// The video page's comment form: a picture alone is a comment (the owner, 11-12), words alone are, nothing is not.
++const assert = require('assert');
++const { readCommentForm } = require('../server/comments/form');
++async function reqOf(fields) {
++    const fd = new FormData();
++    for (const [k, v] of fields) fd.append(k, v);
++    const r = new Request('http://x/', { method: 'POST', body: fd });
++    const body = Buffer.from(await r.arrayBuffer());
++    return { body, get: (h) => (h.toLowerCase() === 'content-type' ? r.headers.get('content-type') : null) };
++}
++(async () => {
++    const pic = new Blob([Buffer.from('png bytes')], { type: 'image/png' });
++    const onlyPic = await readCommentForm(await reqOf([['message', ''], ['files', new File([pic], 'a.png', { type: 'image/png' })]]));
++    assert.deepStrictEqual([onlyPic.message, onlyPic.files.length], ['', 1], 'a picture with no caption');
++    const words = await readCommentForm(await reqOf([['message', '  nice  ']]));
++    assert.deepStrictEqual([words.message, words.files.length], ['nice', 0]);
++    await assert.rejects(readCommentForm(await reqOf([['message', '   ']])), /Write something or attach a picture/);
++    await assert.rejects(readCommentForm(await reqOf([['message', 'x'.repeat(2001)]])), /up to 2,000 characters/);
++    console.log('comment form: a picture alone, words alone, never nothing');
++})().catch((e) => { console.error(e); process.exit(1); });