Avoid overwriting preexisting preview images unless user selected "overwrite".

- Fixed invalid exceptions.

__init__.py

@@ -433,7 +433,7 @@ async def get_image_extensions(request):
 def download_model_preview(formdata):
     path = formdata.get("path", None)
     if type(path) is not str:
-        raise ("Invalid path!")
+        raise ValueError("Invalid path!")
     path, model_type = search_path_to_system_path(path)
     model_type_extensions = folder_paths_get_supported_pt_extensions(model_type)
     path_without_extension, _ = split_valid_ext(path, model_type_extensions)
@@ -474,32 +474,36 @@ def download_model_preview(formdata):
     else:
         content_type = image.content_type
         if not content_type.startswith("image/"):
-            raise ("Invalid content type!")
+            raise RuntimeError("Invalid content type!")
         image_extension = "." + content_type[len("image/"):]
         if image_extension not in image_extensions:
-            raise ("Invalid extension!")
+            raise RuntimeError("Invalid extension!")
 
         image_path = path_without_extension + image_extension
         if not overwrite and os.path.isfile(image_path):
-            raise ("Image already exists!")
+            raise RuntimeError("Image already exists!")
         file: io.IOBase = image.file
         image_data = file.read()
         with open(image_path, "wb") as f:
             f.write(image_data)
         print("Saved file: " + image_path)
 
-    delete_same_name_files(path_without_extension, preview_extensions, image_extension)
+    if overwrite:
+        delete_same_name_files(path_without_extension, preview_extensions, image_extension)
 
-    # detect and fix wrong file extension
+    # detect (and try to fix) wrong file extension
     image_format = None
     with Image.open(image_path) as image:
         image_format = image.format
     image_dir_and_name, image_ext = os.path.splitext(image_path)
     if not image_format_is_equal(image_format, image_ext):
         corrected_image_path = image_dir_and_name + "." + image_format.lower()
-        os.rename(image_path, corrected_image_path)
-        print("Saved file: " + corrected_image_path)
-        image_path = corrected_image_path
+        if os.path.exists(corrected_image_path) and not overwrite:
+            print("WARNING: '" + image_path + "' has wrong extension!")
+        else:
+            os.rename(image_path, corrected_image_path)
+            print("Saved file: " + corrected_image_path)
+            image_path = corrected_image_path
     return image_path # return in-case need corrected path