fix: detect the real image type of bare base64 generated images (#30359)

Generated images that come back as bare base64 (OpenAI b64_json, Gemini
bytesBase64Encoded and inlineData, Automatic1111) were always stored as
generated-image.png with content type image/png, even when the provider
returned JPEG or WebP, for example with {"output_format": "jpeg"} in the
OpenAI extra params. The image still rendered because browsers read the
bytes, but the download name, the served Content-Type and the type sent
along on a later image edit were wrong.

Bare base64 carries no format, so the type is now read from the bytes with
Pillow, the same way the file already inspects images elsewhere. A response
that is not an image at all now fails the generation instead of storing a
broken png. The file extension comes from the module's own extension map
first, because the Python 3.11 Docker image has no mime database entry for
WebP and would otherwise name the file generated-imageNone.

Fixes #29948

backend/open_webui/routers/images.py

@@ -509,8 +509,9 @@ async def get_image_data(data: str, headers=None, trusted_base_url: str | None =
                 mime_type = header.split(';')[0].lstrip('data:')
                 img_data = base64.b64decode(encoded)
             else:
-                mime_type = 'image/png'
                 img_data = base64.b64decode(data)
+                with Image.open(io.BytesIO(img_data)) as image:
+                    mime_type = Image.MIME.get(image.format, 'image/png')
             return img_data, mime_type
     except Exception as e:
         log.exception(f'Error loading image data: {e}')
@@ -520,7 +521,7 @@ async def get_image_data(data: str, headers=None, trusted_base_url: str | None =
 async def upload_image(request, image_data, content_type, metadata, user, db=None):
     if image_data is None or content_type is None:
         raise ValueError('Failed to retrieve image data from the generation backend')
-    image_format = mimetypes.guess_extension(content_type)
+    image_format = IMAGE_FILE_EXTENSIONS.get(content_type.lower()) or mimetypes.guess_extension(content_type) or '.png'
     file = UploadFile(
         file=io.BytesIO(image_data),
         filename=f'generated-image{image_format}',  # will be converted to a unique ID on upload_file