diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index 91d1dafe64..9b15699e4d 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -53,6 +53,9 @@ from ._base import FileInfo
logger = logging.getLogger(__name__)
+_charset_match = re.compile(br"<\s*meta[^>]*charset\s*=\s*([a-z0-9-]+)", flags=re.I)
+_content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
+
class PreviewUrlResource(Resource):
isLeaf = True
@@ -223,15 +226,25 @@ class PreviewUrlResource(Resource):
with open(media_info['filename'], 'rb') as file:
body = file.read()
- # clobber the encoding from the content-type, or default to utf-8
- # XXX: this overrides any <meta/> or XML charset headers in the body
- # which may pose problems, but so far seems to work okay.
- match = re.match(
- r'.*; *charset="?(.*?)"?(;|$)',
- media_info['media_type'],
- re.I
- )
- encoding = match.group(1) if match else "utf-8"
+ encoding = None
+
+ # Let's try and figure out if it has an encoding set in a meta tag.
+ # Limit it to the first 1kb, since it ought to be in the meta tags
+ # at the top.
+ match = _charset_match.search(body[:1000])
+
+ # If we find a match, it should take precedence over the
+ # Content-Type header, so set it here.
+ if match:
+ encoding = match.group(1).decode('ascii')
+
+ # If we don't find a match, we'll look at the HTTP Content-Type, and
+ # if that doesn't exist, we'll fall back to UTF-8.
+ if not encoding:
+ match = _content_type_match.match(
+ media_info['media_type']
+ )
+ encoding = match.group(1) if match else "utf-8"
og = decode_and_calc_og(body, media_info['uri'], encoding)
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