feat(web): preserve file MIME type in the .pbf.json container

The browser File API gives file.type for free, so the Encode tab now records it as
an optional 'mime' field (omitted when the browser reports none). On decode the
metadata panel shows it and, more usefully, the download Blob uses it instead of
generic application/octet-stream so images/pdfs/etc. open correctly. CLIs ignore
and omit it (no MIME detection dependency); the field is additive/backward-compatible.

Author: pmarreck

index.html

@@ -340,7 +340,7 @@ <h1>PrintableBinary Encoder/Decoder</h1>
         if (fmt === 'container') {
           updateProgress(40);
           const bytes = new Uint8Array(await file.arrayBuffer());
-          const container = encoder.encodeToContainer(bytes, { filename: file.name, modified_ms: file.lastModified });
+          const container = encoder.encodeToContainer(bytes, { filename: file.name, modified_ms: file.lastModified, mime: file.type || undefined });
           const json = JSON.stringify(container, null, 2);
           updateProgress(100);
           const dur = ((performance.now() - start) / 1000).toFixed(2);
@@ -385,6 +385,7 @@ <h1>PrintableBinary Encoder/Decoder</h1>
         if (m.created_ms != null) html += `<div class="mp-row"><span class="mp-key">created</span> ${new Date(m.created_ms).toLocaleString()}</div>`;
         if (m.mode != null) html += `<div class="mp-row"><span class="mp-key">mode</span> ${escapeHtml(String(m.mode))}</div>`;
         if (m.owner != null) html += `<div class="mp-row"><span class="mp-key">owner</span> ${escapeHtml(String(m.owner))}${m.group != null ? ':' + escapeHtml(String(m.group)) : ''}</div>`;
+        if (m.mime) html += `<div class="mp-row"><span class="mp-key">type</span> ${escapeHtml(String(m.mime))}</div>`;
         html += `<div class="mp-row"><span class="mp-key">integrity</span> <span class="mp-ok">✓ CRC-32 verified</span></div>`;
       } else {
         html += `<div class="mp-title">Raw printable-binary → decoded</div>`;
@@ -410,7 +411,7 @@ <h1>PrintableBinary Encoder/Decoder</h1>
         renderMeta(res);
         decStats.textContent = `Decoded ${formatFileSize(res.bytes.length)} in ${dur}s — ready to download as “${res.filename}”.`;
         decDownloadBtn.disabled = false;
-        decDownloadBtn.payload = { bytes: res.bytes, filename: res.filename, mime: 'application/octet-stream' };
+        decDownloadBtn.payload = { bytes: res.bytes, filename: res.filename, mime: res.meta.mime || 'application/octet-stream' };
         decClearBtn.disabled = false;
       } catch (err) {
         metaPanel.classList.remove('visible');

js/printable_binary.js

@@ -713,7 +713,7 @@ class PrintableBinary {
       crc32: this.crc32hex(bytes),
       crc32_encoded: this.crc32hex(dataBytes),
     };
-    for (const k of ["modified_ms", "created_ms", "mode", "owner", "group"]) {
+    for (const k of ["modified_ms", "created_ms", "mode", "owner", "group", "mime"]) {
       if (meta[k] !== undefined && meta[k] !== null) container[k] = meta[k];
     }
     // `data` is appended LAST so all metadata sits up front (JS object key
@@ -760,7 +760,7 @@ class PrintableBinary {
       }
     }
     const meta = {};
-    for (const k of ["filename", "modified_ms", "created_ms", "mode", "owner", "group"]) {
+    for (const k of ["filename", "modified_ms", "created_ms", "mode", "owner", "group", "mime"]) {
       if (c[k] !== undefined && c[k] !== null) meta[k] = c[k];
     }
     return { bytes, meta };

test/js/test_printable_binary.js

@@ -316,7 +316,7 @@ console.log('\n--- Container (printable-binary-file.json) Tests ---');
   // Container round-trip (MFIC inverse-pair oracle): bytes + metadata identical
   const bytes = new Uint8Array([0, 1, 2, 255, 254, 65, 66, 10, 9, 0x7f, 0x80, 0]);
   const container = encoder.encodeToContainer(bytes, {
-    filename: "x.bin", modified_ms: 1719430000000, mode: "0644",
+    filename: "x.bin", modified_ms: 1719430000000, mode: "0644", mime: "image/png",
   });
   assertEquals(container.format, "printable-binary-file", "container has format discriminator");
   assertEquals(container.version, 1, "container version 1");
@@ -328,6 +328,7 @@ console.log('\n--- Container (printable-binary-file.json) Tests ---');
   assertEquals(meta.filename, "x.bin", "container round-trip: filename restored");
   assertEquals(meta.modified_ms, 1719430000000, "container round-trip: modified_ms restored");
   assertEquals(meta.mode, "0644", "container round-trip: mode restored");
+  assertEquals(meta.mime, "image/png", "container round-trip: mime restored");
 
   // Round-trip through a JSON string too (the on-disk form)
   const rt = encoder.decodeFromContainer(JSON.stringify(container));