Change Literal to str, because PyArrow doesn't know about literals

app/api/models.py

@@ -41,7 +41,7 @@ class BinaryMask(pydantic.BaseModel):
     maximum compactness. You can use the ``BinaryMask.from_numpy()``
     function to convert a 0-1 numpy array to a BinaryMask.
     """
-    mediaType: Literal["image/png"] = pydantic.Field(
+    mediaType: str = pydantic.Field(
         description="The IETF Media Type (MIME type) of the data."
     )
     data: str = pydantic.Field(
@@ -51,6 +51,13 @@ class BinaryMask(pydantic.BaseModel):
         pattern=r"^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/][AQgw]==|[A-Za-z0-9+/]{2}[AEIMQUYcgkosw048]=)?$",
     )
 
+    @pydantic.field_validator("mediaType", mode="after")
+    @classmethod
+    def validate_mediaType(cls, value: str) -> str:
+        if value != "image/png":
+            raise ValueError(".mediaType must be 'image/png'")
+        return value
+
     @staticmethod
     def from_numpy(mask: NDArray[np.uint8]) -> BinaryMask:
         """Convert a 0-1 numpy array to a BinaryMask.