diff --git a/lib/plugin-sandbox/host-api.ts b/lib/plugin-sandbox/host-api.ts
index d489b3249..b13569294 100644
--- a/lib/plugin-sandbox/host-api.ts
+++ b/lib/plugin-sandbox/host-api.ts
@@ -232,7 +232,38 @@ function isApiPostPathAllowed(path: string, allowlist: readonly string[]): boole
   return false;
 }
 
-async function doHttpPost(plugin: InstalledPlugin, path: string, body: unknown): Promise<{ ok: boolean; status: number; data: unknown }> {
+interface PluginHttpPostOptions {
+  headers?: Record<string, string>;
+}
+
+/**
+ * Namespace a plugin must use for its own upload metadata headers. Anything
+ * outside it (and `Content-Type`) is refused, so a plugin can never reach the
+ * credential headers the host attaches to the request.
+ */
+const PLUGIN_HEADER_PREFIX = 'x-plugin-';
+
+function applyPluginUploadHeaders(
+  provided: Record<string, string> | undefined,
+  target: Record<string, string>,
+): void {
+  for (const [name, value] of Object.entries(provided ?? {})) {
+    const lower = name.toLowerCase();
+    if (lower !== 'content-type' && !lower.startsWith(PLUGIN_HEADER_PREFIX)) {
+      throw new Error(
+        `Header ${name} is not allowed on a binary plugin upload (use Content-Type or an X-Plugin-* header)`,
+      );
+    }
+    target[name] = String(value);
+  }
+}
+
+async function doHttpPost(
+  plugin: InstalledPlugin,
+  path: string,
+  body: unknown,
+  options?: PluginHttpPostOptions,
+): Promise<{ ok: boolean; status: number; data: unknown }> {
   if (typeof path !== 'string' || !path.startsWith('/api/')) {
     throw new Error('path must start with /api/');
   }
@@ -250,7 +281,25 @@ async function doHttpPost(plugin: InstalledPlugin, path: string, body: unknown):
     throw new Error(`Path ${url.pathname} not in plugin apiPostPaths allowlist`);
   }
   const { client } = useAuthStore.getState();
-  const headers: Record<string, string> = { 'Content-Type': 'application/json' };
+  const headers: Record<string, string> = {};
+  let requestBody: BodyInit;
+
+  if (body instanceof Blob) {
+    // Binary upload: the plugin owns Content-Type and any X-Plugin-* metadata
+    // the receiving route needs. Stock behaviour for every other body type is
+    // unchanged - it is still serialized as JSON.
+    applyPluginUploadHeaders(options?.headers, headers);
+    const hasContentType = Object.keys(headers).some(h => h.toLowerCase() === 'content-type');
+    if (!hasContentType && body.type) {
+      headers['Content-Type'] = body.type;
+    }
+    requestBody = body;
+  } else {
+    headers['Content-Type'] = 'application/json';
+    requestBody = JSON.stringify(body);
+  }
+
+  // Applied last so plugin-supplied headers can never override credentials.
   if (client) {
     headers['Authorization'] = client.getAuthHeader();
     headers['X-JMAP-Username'] = client.getUsername();
@@ -258,7 +307,7 @@ async function doHttpPost(plugin: InstalledPlugin, path: string, body: unknown):
   const res = await apiFetch(url.pathname + url.search, {
     method: 'POST',
     headers,
-    body: JSON.stringify(body),
+    body: requestBody,
   });
   const data = await res.json().catch(() => null);
   return { ok: res.ok, status: res.status, data };
@@ -795,7 +844,7 @@ export async function dispatchApiCall(
     case 'toast.info':    appToast.info(String(args[0] ?? '')); return undefined;
     case 'toast.warning': appToast.warning(String(args[0] ?? '')); return undefined;
 
-    case 'http.post':  return doHttpPost(plugin, args[0] as string, args[1]);
+    case 'http.post':  return doHttpPost(plugin, args[0] as string, args[1], args[2] as PluginHttpPostOptions | undefined);
     case 'http.fetch': return doHttpFetch(plugin, args[0] as string, args[1] as PluginFetchInit | undefined);
 
     case 'jmap.fetchBlob': return doJmapFetchBlob(args[0] as string, args[1] as { name?: string; type?: string } | undefined);
diff --git a/lib/plugin-sandbox/runtime.tsx b/lib/plugin-sandbox/runtime.tsx
index bb8c2b3e1..9b57b2284 100644
--- a/lib/plugin-sandbox/runtime.tsx
+++ b/lib/plugin-sandbox/runtime.tsx
@@ -187,7 +187,11 @@ function buildPluginApi(manifest: PluginManifest) {
       logout: () => callApi('user.logout', []),
     },
     http: {
-      post: (path: string, body: Record<string, unknown>) => callApi('http.post', [path, body]),
+      // A Blob/File body is sent as a binary request; options.headers may then
+      // carry Content-Type and X-Plugin-* metadata for the receiving route.
+      // Any other body keeps the stock JSON behaviour.
+      post: (path: string, body: Record<string, unknown> | Blob, options?: { headers?: Record<string, string> }) =>
+        callApi('http.post', [path, body, options]),
       fetch: (url: string, init?: unknown) => callApi('http.fetch', [url, init]),
     },
     // Privileged-tier only (same-origin plugins). Calls throw for untrusted
@@ -225,6 +229,7 @@ function buildPluginApi(manifest: PluginManifest) {
     /**
      * Used to alterate files before they are uploaded to server.
      * Edited files are saved on indexedDB and remove once the upload to server begins.
+     * `get` needs the email:blob-read permission, `save` needs email:blob-write.
      */
     upfiles: {
       save: (formerFileId:string, file:File) =>