diff --git a/changelog.d/9788.bugfix b/changelog.d/9788.bugfix
new file mode 100644
index 000000000000..edb58fbd5b37
--- /dev/null
+++ b/changelog.d/9788.bugfix
@@ -0,0 +1 @@
+Fix thumbnail generation for some sites with non-standard content types. Contributed by @rkfg.
diff --git a/synapse/config/repository.py b/synapse/config/repository.py
index 061c4ec83fc7..508d8754833f 100644
--- a/synapse/config/repository.py
+++ b/synapse/config/repository.py
@@ -71,6 +71,7 @@ def parse_thumbnail_requirements(thumbnail_sizes):
         jpeg_thumbnail = ThumbnailRequirement(width, height, method, "image/jpeg")
         png_thumbnail = ThumbnailRequirement(width, height, method, "image/png")
         requirements.setdefault("image/jpeg", []).append(jpeg_thumbnail)
+        requirements.setdefault("image/jpg", []).append(jpeg_thumbnail)
         requirements.setdefault("image/webp", []).append(jpeg_thumbnail)
         requirements.setdefault("image/gif", []).append(png_thumbnail)
         requirements.setdefault("image/png", []).append(png_thumbnail)
diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index 0c041b542d4d..5c4d33ee07d6 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -468,6 +468,9 @@ async def _download_remote_file(
         return media_info
 
     def _get_thumbnail_requirements(self, media_type):
+        scpos = media_type.find(";")
+        if scpos > 0:
+            media_type = media_type[:scpos]
         return self.thumbnail_requirements.get(media_type, ())
 
     def _generate_thumbnail(