diff --git a/src/api/blob.ts b/src/api/blob.ts
index 1d6926a4..4bda205d 100644
--- a/src/api/blob.ts
+++ b/src/api/blob.ts
@@ -13,17 +13,20 @@ export async function uploadBlob(
     encodeURIComponent(jmapClient.accountId),
   );
 
-  // Read the source via the file-system API so this works for both `file://`
-  // (image picker, iOS document picker) and `content://` (Android SAF) URIs.
+  // Read via the file-system API so this works for both `file://` (image
+  // picker, iOS document picker) and `content://` (Android SAF) URIs. We pass
+  // the underlying ArrayBuffer rather than the Uint8Array view because some
+  // RN transports stringify typed-array bodies, which silently produces an
+  // empty/garbage upload.
   const bytes = await new File(uri).bytes();
 
   const response = await fetch(uploadUrl, {
     method: 'POST',
     headers: {
-      'Content-Type': type,
-      Authorization: (jmapClient as any).authHeader,
+      'Content-Type': type || 'application/octet-stream',
+      Authorization: jmapClient.authHeader,
     },
-    body: bytes as unknown as BodyInit,
+    body: bytes.buffer as ArrayBuffer,
   });
 
   if (!response.ok) {
@@ -66,8 +69,8 @@ export function getDownloadUrl(
   if (!session) throw new Error('Not connected');
 
   return session.downloadUrl
-    .replace('{accountId}', jmapClient.accountId)
-    .replace('{blobId}', blobId)
+    .replace('{accountId}', encodeURIComponent(jmapClient.accountId))
+    .replace('{blobId}', encodeURIComponent(blobId))
     .replace('{name}', encodeURIComponent(name ?? 'download'))
     .replace('{type}', encodeURIComponent(type ?? 'application/octet-stream'));
 }
diff --git a/src/api/jmap-client.ts b/src/api/jmap-client.ts
index eac21ca7..befa8e05 100644
--- a/src/api/jmap-client.ts
+++ b/src/api/jmap-client.ts
@@ -53,7 +53,10 @@ export class JMAPClient {
 
   // ── Authentication ────────────────────────────────────
 
-  private get authHeader(): string {
+  // Public so blob upload/download helpers (which can't go through the JMAP
+  // request body) can fetch the same Authorization header the rest of the
+  // client uses. The value is recomputed each access and never cached.
+  get authHeader(): string {
     if (!this.credentials) throw new Error('No credentials');
     if (this.credentials.accessToken) {
       return `Bearer ${this.credentials.accessToken}`;
@@ -161,17 +164,18 @@ export class JMAPClient {
   // JMAP servers often self-report localhost/container-internal hostnames in
   // apiUrl/downloadUrl/etc. that are unreachable from mobile clients (e.g.
   // the Android emulator can't resolve the host's "localhost").
+  //
+  // Splits via plain string indexing rather than `new URL()` because RN's
+  // URL polyfill mutates inputs (appends trailing slashes, normalises
+  // characters) and would corrupt the RFC 6570 templates {accountId}/{blobId}
+  // before the caller has a chance to substitute values into them.
   private rewriteSessionUrls(session: JMAPSession, serverUrl: string): JMAPSession {
+    const serverOrigin = this.extractOrigin(serverUrl);
     const rewrite = (url: string | undefined): string | undefined => {
       if (!url) return url;
-      try {
-        const parsed = new URL(url);
-        const server = new URL(serverUrl);
-        if (parsed.origin === server.origin) return url;
-        return server.origin + parsed.pathname + parsed.search + parsed.hash;
-      } catch {
-        return url;
-      }
+      const origin = this.extractOrigin(url);
+      if (!origin || !serverOrigin || origin === serverOrigin) return url;
+      return serverOrigin + url.slice(origin.length);
     };
     return {
       ...session,
@@ -182,6 +186,11 @@ export class JMAPClient {
     };
   }
 
+  private extractOrigin(url: string): string | null {
+    const m = url.match(/^(https?:\/\/[^/?#]+)/i);
+    return m ? m[1] : null;
+  }
+
   // Reset in-memory state without touching persisted credentials (used on
   // account switch so the active account's stored creds remain available).
   reset(): void {
diff --git a/src/api/push.ts b/src/api/push.ts
index 65d294ed..29f9249d 100644
--- a/src/api/push.ts
+++ b/src/api/push.ts
@@ -134,7 +134,7 @@ export function connectEventSource(
     : require('react-native-sse').default;
 
   const es = new EventSourceImpl(url, {
-    headers: { Authorization: (jmapClient as any).authHeader },
+    headers: { Authorization: jmapClient.authHeader },
   });
 
   const handler = (event: any) => {
diff --git a/src/lib/email-export.ts b/src/lib/email-export.ts
index 594ee65a..8d09e314 100644
--- a/src/lib/email-export.ts
+++ b/src/lib/email-export.ts
@@ -7,9 +7,7 @@ import { getDownloadUrl } from '../api/blob';
 const RFC822 = 'message/rfc822';
 
 function authHeader(): string {
-  const h = (jmapClient as unknown as { authHeader?: string }).authHeader;
-  if (!h) throw new Error('Not connected');
-  return h;
+  return jmapClient.authHeader;
 }
 
 function safeAttachmentName(name: string | undefined, type: string | undefined): string {