Provide more info why we don't have any thumbnails to serve (#13038)

Fix https://github.com/matrix-org/synapse/issues/13016

## New error code and status

### Before

Previously, we returned a `404` for `/thumbnail` which isn't even in the spec.

```json
{
  "errcode": "M_NOT_FOUND",
  "error": "Not found [b'hs1', b'tefQeZhmVxoiBfuFQUKRzJxc']"
}
```

### After

What does the spec say?

> 400: The request does not make sense to the server, or the server cannot thumbnail the content. For example, the client requested non-integer dimensions or asked for negatively-sized images.
>
> *-- https://spec.matrix.org/v1.1/client-server-api/#get_matrixmediav3thumbnailservernamemediaid*

Now with this PR, we respond with a `400` when we don't have thumbnails to serve and we explain why we might not have any thumbnails.

```json
{
    "errcode": "M_UNKNOWN",
    "error": "Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)",
}
```

> Cannot find any thumbnails for the requested media ([b'example.com', b'12345']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)


---

We still respond with a 404 in many other places. But we can iterate on those later and maybe keep some in some specific places after spec updates/clarification: https://github.com/matrix-org/matrix-spec/issues/1122

We can also iterate on the bugs where Synapse doesn't thumbnail when it should in other issues/PRs.

synapse/config/repository.py

@@ -42,6 +42,18 @@ THUMBNAIL_SIZE_YAML = """\
         #    method: %(method)s
 """
 
+# A map from the given media type to the type of thumbnail we should generate
+# for it.
+THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP = {
+    "image/jpeg": "jpeg",
+    "image/jpg": "jpeg",
+    "image/webp": "jpeg",
+    # Thumbnails can only be jpeg or png. We choose png thumbnails for gif
+    # because it can have transparency.
+    "image/gif": "png",
+    "image/png": "png",
+}
+
 HTTP_PROXY_SET_WARNING = """\
 The Synapse config url_preview_ip_range_blacklist will be ignored as an HTTP(s) proxy is configured."""
 
@@ -79,13 +91,22 @@ def parse_thumbnail_requirements(
         width = size["width"]
         height = size["height"]
         method = size["method"]
-        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)
+
+        for format, thumbnail_format in THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP.items():
+            requirement = requirements.setdefault(format, [])
+            if thumbnail_format == "jpeg":
+                requirement.append(
+                    ThumbnailRequirement(width, height, method, "image/jpeg")
+                )
+            elif thumbnail_format == "png":
+                requirement.append(
+                    ThumbnailRequirement(width, height, method, "image/png")
+                )
+            else:
+                raise Exception(
+                    "Unknown thumbnail mapping from %s to %s. This is a Synapse problem, please report!"
+                    % (format, thumbnail_format)
+                )
     return {
         media_type: tuple(thumbnails) for media_type, thumbnails in requirements.items()
     }