MM-22057: Limit incoming request bodies (#13827)

* MM-22057: Limit incoming request bodies

Set the max request body size to be equal to MaxFileSize.
Ideally, non-file request bodies should be smaller than file request bodies,
but we don't have a clean way to identify all file upload handlers.

There shouldn't be any valid request which exceeds the max file upload size.
So this is a safe global limit to apply.

* Fix tests

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

app/file.go

@@ -647,6 +647,11 @@ func (a *App) UploadFileX(channelId, name string, input io.Reader,
 func (t *uploadFileTask) readAll() *model.AppError {
 	_, err := t.buf.ReadFrom(t.limitedInput)
 	if err != nil {
+		// Ugly hack: the error is not exported from net/http.
+		if err.Error() == "http: request body too large" {
+			return t.newAppError("api.file.upload_file.too_large_detailed.app_error",
+				"", http.StatusRequestEntityTooLarge, "Length", t.buf.Len(), "Limit", t.limit)
+		}
 		return t.newAppError("api.file.upload_file.read_request.app_error",
 			err.Error(), http.StatusBadRequest)
 	}