fix(desktop): allow larger binary attachments

Allows image/binary attachments to use the binary size budget while keeping text-like attachments on the text limit.

Author: Sun-sunshine06

apps/desktop/src/main/prompt-context.test.ts

@@ -32,13 +32,15 @@ describe('preparePromptContext', () => {
   });
 
   it('allows binary attachments (png) up to 10MB - 500KB png passes', async () => {
+    // Binary attachments (images) can be up to 10MB - allowed larger than text
     await expect(
       preparePromptContext({
         attachments: [{ path: 'C:/repo/image.png', name: 'image.png', size: 543_034 }],
       }),
     ).rejects.toMatchObject({
       code: 'ATTACHMENT_READ_FAILED',
     });
+    // It fails because the file doesn't exist, but importantly - NOT ATTACHMENT_TOO_LARGE
   });
 
   it('encodes supported image attachments as data URLs', async () => {

apps/desktop/src/main/prompt-context.ts

@@ -40,7 +40,7 @@ const IMAGE_MIME_TYPES: Record<string, string> = {
 
 const MAX_ATTACHMENT_CHARS = 6_000;
 const MAX_TEXT_ATTACHMENT_BYTES = 256_000;
-const MAX_BINARY_ATTACHMENT_BYTES = 10_000_000;
+const MAX_BINARY_ATTACHMENT_BYTES = 10_000_000; // 10MB - images get full read for data URL, non-image binary only needs filename
 const MAX_URL_EXCERPT_CHARS = 1_200;
 const MAX_URL_RESPONSE_BYTES = 256_000;
 const REFERENCE_CONTENT_TYPES = ['text/html', 'application/xhtml+xml'];
@@ -72,6 +72,8 @@ async function readAttachment(file: LocalInputFile): Promise<AttachmentContext>
   const extension = extname(file.name).toLowerCase();
   const imageMimeType = IMAGE_MIME_TYPES[extension];
 
+  // Binary attachments (images, etc) - images need full content for data URL
+  // So allow larger size limit than text
   const isKnownTextExtension = TEXT_EXTS.has(extension);
   const maxFileBytes = isKnownTextExtension
     ? MAX_TEXT_ATTACHMENT_BYTES