diff --git a/jam-ui/src/services/attachmentValidation.js b/jam-ui/src/services/attachmentValidation.js
new file mode 100644
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+++ b/jam-ui/src/services/attachmentValidation.js
@@ -0,0 +1,127 @@
+/**
+ * Attachment Validation Service
+ *
+ * Client-side file validation for session attachment uploads
+ * - Validates file size (10 MB limit)
+ * - Validates file type (extension whitelist)
+ * - Determines attachment type (audio vs notation)
+ * - Formats file sizes for display
+ */
+
+/**
+ * Maximum file size: 10 MB
+ * Matches legacy client-side validation
+ */
+export const MAX_FILE_SIZE = 10 * 1024 * 1024; // 10 MB in bytes
+
+/**
+ * Allowed file extensions
+ * Note: .mp3 support pending backend decision (see Phase 12 docs)
+ */
+export const ALLOWED_EXTENSIONS = [
+  '.pdf', '.xml', '.mxl', '.txt',           // Notation
+  '.png', '.jpg', '.jpeg', '.gif',          // Images
+  '.mp3', '.wav'                            // Audio (pending backend mp3 support)
+];
+
+/**
+ * Backend-supported extensions (from MusicNotationUploader)
+ * Used for warning messages if frontend allows extensions backend doesn't
+ */
+export const BACKEND_SUPPORTED_EXTENSIONS = [
+  '.pdf', '.xml', '.mxl', '.txt',
+  '.png', '.jpg', '.jpeg', '.gif',
+  '.wav', '.flac', '.ogg', '.aiff', '.aifc', '.au'
+];
+
+/**
+ * Audio file extensions (for attachment_type determination)
+ */
+export const AUDIO_EXTENSIONS = ['.mp3', '.wav', '.flac', '.ogg', '.aiff', '.aifc', '.au'];
+
+/**
+ * Validate file size
+ * @param {File} file - File object from input
+ * @param {number} [maxSizeBytes=MAX_FILE_SIZE] - Maximum allowed size
+ * @returns {Object} { valid: boolean, error: string|null }
+ */
+export const validateFileSize = (file, maxSizeBytes = MAX_FILE_SIZE) => {
+  if (file.size > maxSizeBytes) {
+    const maxSizeMB = Math.floor(maxSizeBytes / (1024 * 1024));
+    return {
+      valid: false,
+      error: `File exceeds ${maxSizeMB} MB limit`
+    };
+  }
+  return { valid: true, error: null };
+};
+
+/**
+ * Validate file type by extension
+ * @param {File} file - File object from input
+ * @param {string[]} [allowedTypes=ALLOWED_EXTENSIONS] - Array of allowed extensions
+ * @returns {Object} { valid: boolean, error: string|null }
+ */
+export const validateFileType = (file, allowedTypes = ALLOWED_EXTENSIONS) => {
+  const fileName = file.name.toLowerCase();
+  const hasAllowedExtension = allowedTypes.some(ext => fileName.endsWith(ext));
+
+  if (!hasAllowedExtension) {
+    return {
+      valid: false,
+      error: `File type not allowed. Supported: ${allowedTypes.join(', ')}`
+    };
+  }
+
+  return { valid: true, error: null };
+};
+
+/**
+ * Get attachment type from filename extension
+ * @param {string} filename - File name with extension
+ * @returns {string} 'notation' | 'audio'
+ */
+export const getAttachmentType = (filename) => {
+  const ext = '.' + filename.split('.').pop().toLowerCase();
+  return AUDIO_EXTENSIONS.includes(ext) ? 'audio' : 'notation';
+};
+
+/**
+ * Comprehensive file validation (size + type)
+ * @param {File} file - File object from input
+ * @returns {Object} { valid: boolean, error: string|null, warnings: string[] }
+ */
+export const validateFile = (file) => {
+  const warnings = [];
+
+  // Validate size first (fail fast)
+  const sizeValidation = validateFileSize(file);
+  if (!sizeValidation.valid) {
+    return { valid: false, error: sizeValidation.error, warnings };
+  }
+
+  // Validate type
+  const typeValidation = validateFileType(file);
+  if (!typeValidation.valid) {
+    return { valid: false, error: typeValidation.error, warnings };
+  }
+
+  // Check if backend supports this type (warning only)
+  const ext = '.' + file.name.split('.').pop().toLowerCase();
+  if (!BACKEND_SUPPORTED_EXTENSIONS.includes(ext)) {
+    warnings.push(`File type ${ext} may not be supported by server`);
+  }
+
+  return { valid: true, error: null, warnings };
+};
+
+/**
+ * Format file size for display
+ * @param {number} bytes - File size in bytes
+ * @returns {string} Formatted size (e.g., "2.5 MB")
+ */
+export const formatFileSize = (bytes) => {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+};