Address PR #192 review comments

- Fix TOCTOU race condition by removing os.path.exists() check before file open
- Move imports (base64, binascii, os, utils.file_types) to top of file
- Replace broad Exception catch with specific binascii.Error for base64 decoding
- Maintain proper error handling and test compatibility

providers/base.py

@@ -1,11 +1,16 @@
 """Base model provider interface and data classes."""
 
+import base64
+import binascii
 import logging
+import os
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from enum import Enum
 from typing import Any, Optional
 
+from utils.file_types import IMAGES, get_image_mime_type
+
 logger = logging.getLogger(__name__)
 
 
@@ -459,11 +464,6 @@ class ModelProvider(ABC):
             # Validate with custom size limit
             image_bytes, mime_type = provider.validate_image("/path/to/image.jpg", max_size_mb=10.0)
         """
-        import base64
-        import os
-
-        from utils.file_types import IMAGES, get_image_mime_type
-
         # Use default if not specified
         if max_size_mb is None:
             max_size_mb = self.DEFAULT_MAX_IMAGE_SIZE_MB
@@ -484,12 +484,18 @@ class ModelProvider(ABC):
             # Decode base64 data
             try:
                 image_bytes = base64.b64decode(data)
-            except Exception as e:
+            except binascii.Error as e:
                 raise ValueError(f"Invalid base64 data: {e}")
         else:
             # Handle file path
-            if not os.path.exists(image_path):
+            # Read file first to check if it exists
+            try:
+                with open(image_path, "rb") as f:
+                    image_bytes = f.read()
+            except FileNotFoundError:
                 raise ValueError(f"Image file not found: {image_path}")
+            except Exception as e:
+                raise ValueError(f"Failed to read image file: {e}")
 
             # Validate extension
             ext = os.path.splitext(image_path)[1].lower()
@@ -499,13 +505,6 @@ class ModelProvider(ABC):
             # Get MIME type
             mime_type = get_image_mime_type(ext)
 
-            # Read file
-            try:
-                with open(image_path, "rb") as f:
-                    image_bytes = f.read()
-            except Exception as e:
-                raise ValueError(f"Failed to read image file: {e}")
-
         # Validate size
         size_mb = len(image_bytes) / (1024 * 1024)
         if size_mb > max_size_mb: