diff --git a/synapse/http/client.py b/synapse/http/client.py
index 49a2f3fb39ed..abe827a85cf4 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -694,7 +694,7 @@ async def get_file(
         output_stream: BinaryIO,
         max_size: Optional[int] = None,
         headers: Optional[RawHeaders] = None,
-        is_allowed_content_type: Optional[Callable[[bytes], bool]] = None,
+        is_allowed_content_type: Optional[Callable[[str], bool]] = None,
     ) -> Tuple[int, Dict[bytes, List[bytes]], str, int]:
         """GETs a file from a given URL
         Args:
@@ -735,7 +735,7 @@ async def get_file(
 
         if is_allowed_content_type and b"Content-Type" in resp_headers:
             content_type = resp_headers[b"Content-Type"][0]
-            if not is_allowed_content_type(content_type):
+            if not is_allowed_content_type(content_type.decode("ascii")):
                 raise SynapseError(
                     HTTPStatus.BAD_GATEWAY,
                     (
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index 9dd7425929f6..8d3d1e54dc9f 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -764,14 +764,8 @@ def _is_json(content_type: str) -> bool:
     return content_type.lower().startswith("application/json")
 
 
-def _is_previewable(content_type: bytes) -> bool:
+def _is_previewable(content_type: str) -> bool:
     """Returns True for content types for which we will perform URL preview and False
     otherwise."""
 
-    content_type = content_type.lower()
-    return (
-        content_type.startswith(b"text/html")
-        or content_type.startswith(b"application/xhtml")
-        or content_type.startswith(b"image/")
-        or content_type.startswith(b"application/json")
-    )
+    return _is_html(content_type) or _is_media(content_type) or _is_json(content_type)