diff --git a/astrbot/dashboard/services/chat_service.py b/astrbot/dashboard/services/chat_service.py
index 36b0700b92..a1bd86883d 100644
--- a/astrbot/dashboard/services/chat_service.py
+++ b/astrbot/dashboard/services/chat_service.py
@@ -36,6 +36,13 @@
 
 SSE_HEARTBEAT = ": heartbeat\n\n"
 CHAT_RUN_SUBSCRIBER_QUEUE_SIZE = 256
+WEBCHAT_IMAGE_MIME_TYPES = {
+    ".jpg": "image/jpeg",
+    ".jpeg": "image/jpeg",
+    ".png": "image/png",
+    ".gif": "image/gif",
+    ".webp": "image/webp",
+}
 
 
 def sanitize_upload_filename(filename: str | None) -> str:
@@ -504,7 +511,6 @@ def __init__(
         self.webchat_img_dir = os.path.join(get_astrbot_data_path(), "webchat", "imgs")
         os.makedirs(self.attachments_dir, exist_ok=True)
 
-        self.supported_imgs = ["jpg", "jpeg", "png", "gif", "webp"]
         self.conv_mgr = core_lifecycle.conversation_manager
         self.platform_history_mgr = core_lifecycle.platform_message_history_manager
         self.umop_config_router = core_lifecycle.umop_config_router
@@ -557,8 +563,8 @@ async def resolve_webchat_file(
         filename_ext = file_path.suffix.lower()
         if filename_ext == ".wav":
             return str(file_path), "audio/wav"
-        if filename_ext[1:] in self.supported_imgs:
-            return str(file_path), "image/jpeg"
+        if filename_ext in WEBCHAT_IMAGE_MIME_TYPES:
+            return str(file_path), WEBCHAT_IMAGE_MIME_TYPES[filename_ext]
         return str(file_path), None
 
     async def resolve_webchat_file_from_dashboard_query(
diff --git a/tests/unit/test_webchat_upload_image_format.py b/tests/unit/test_webchat_upload_image_format.py
index 14cce65501..7125c6961f 100644
--- a/tests/unit/test_webchat_upload_image_format.py
+++ b/tests/unit/test_webchat_upload_image_format.py
@@ -4,7 +4,10 @@
 import pytest
 from PIL import Image as PILImage
 
-from astrbot.dashboard.services.chat_service import ChatService
+from astrbot.dashboard.services.chat_service import (
+    WEBCHAT_IMAGE_MIME_TYPES,
+    ChatService,
+)
 
 
 @pytest.mark.asyncio
@@ -44,3 +47,35 @@ async def insert_attachment(self, path, type, mime_type):
     assert fake_db.inserted["type"] == "image"
     assert (tmp_path / result["filename"]).exists()
     assert not (tmp_path / "pasted.png").exists()
+
+
+@pytest.mark.parametrize(
+    ("filename", "expected_mime_type"),
+    [(f"photo{ext}", mime_type) for ext, mime_type in WEBCHAT_IMAGE_MIME_TYPES.items()],
+)
+@pytest.mark.asyncio
+async def test_resolve_webchat_file_uses_image_extension_mime_type(
+    tmp_path,
+    monkeypatch,
+    filename,
+    expected_mime_type,
+):
+    monkeypatch.setattr(
+        "astrbot.dashboard.services.chat_service.get_astrbot_data_path",
+        lambda: str(tmp_path),
+    )
+    service = ChatService(
+        SimpleNamespace(),
+        SimpleNamespace(
+            conversation_manager=None,
+            platform_message_history_manager=None,
+            umop_config_router=None,
+        ),
+    )
+    file_path = tmp_path / "attachments" / filename
+    file_path.write_bytes(b"image-bytes")
+
+    resolved_path, mime_type = await service.resolve_webchat_file(filename)
+
+    assert resolved_path == str(file_path.resolve(strict=False))
+    assert mime_type == expected_mime_type