feat(opentrons-ai-server,-opentrons-ai-client): backend attach file (#18994)

# Overview

Closes AUTH-2019

PR implements backend support for Attach file.

Current Flow:
  1. User selects files → handleFileSelect()
  2. When sending message → readFileContent() reads file locally
  3. File content is embedded in fileAttachments array
  4. Sent directly in the chat request body
 
**Attach** 

<img width="1108" height="202" alt="image"
src="https://github.com/user-attachments/assets/20119f7e-74fc-42da-956b-e212eae5bfd4"
/>

**Send**
<img width="1147" height="900" alt="image"
src="https://github.com/user-attachments/assets/d35a8b87-cdad-4850-a2bd-0f54ada6f039"
/>


## Test Plan and Hands on Testing
CI


## Review requests

Attach a Python, CSV, or PDF and ask some questions.

## Risk assessment

Mid

Author: Elyorcv

opentrons-ai-client/src/atoms/AttachedFileItem/index.tsx

@@ -1,46 +1,18 @@
 import { COLORS, Icon } from '@opentrons/components'
 
-import { getFileType } from '/ai-client/resources/utils/fileUtils'
+import {
+  getFileExtension,
+  getFileType,
+} from '/ai-client/resources/utils/fileUtils'
 
 import styles from './attachedfileitem.module.css'
 
 interface AttachedFileItemProps {
-  file: {
-    name: string
-    size?: number
-  }
+  file: File | { name: string; type?: string }
   onRemove?: () => void
   showRemoveButton?: boolean
 }
 
-// Helper to get simple file type labels
-const getSimpleFileTypeLabel = (type: string, fileName: string): string => {
-  // Check for Python files first
-  if (fileName.toLowerCase().endsWith('.py')) {
-    return 'Python file'
-  }
-
-  switch (type) {
-    case 'pdf':
-      return 'PDF file'
-    case 'csv':
-      return 'CSV file'
-    case 'image':
-      return 'Image file'
-    case 'json':
-      return 'JSON file'
-    default:
-      return 'File'
-  }
-}
-
-// Helper to get file extension for display in the icon container
-// eg., example.py, somehting.pdf. first.csv
-const getFileExtension = (fileName: string): string => {
-  const extension = fileName.split('.').pop()?.toLowerCase()
-  return extension != null && extension !== '' ? `.${extension}` : '.file'
-}
-
 export function AttachedFileItem({
   file,
   onRemove,
@@ -54,18 +26,29 @@ export function AttachedFileItem({
       : styles.container_without_remove
   }`
 
+  const getFileTypeLabel = (type: ReturnType<typeof getFileType>): string => {
+    switch (type) {
+      case 'pdf':
+        return 'PDF file'
+      case 'csv':
+        return 'CSV file'
+      case 'python':
+        return 'Python file'
+      default:
+        return 'Unknown file'
+    }
+  }
+
   return (
     <div className={containerClass}>
       <div className={styles.file_icon_container}>
         <div className={styles.file_extension}>
-          {getFileExtension(file.name)}
+          {getFileExtension(file as File)}
         </div>
       </div>
       <div className={styles.file_details_container}>
         <div className={styles.file_name}>{file.name}</div>
-        <div className={styles.file_details}>
-          {getSimpleFileTypeLabel(fileType, file.name)}
-        </div>
+        <div className={styles.file_details}>{getFileTypeLabel(fileType)}</div>
       </div>
       {showRemoveButton && onRemove != null && (
         <button