firefox cannot preload bitmap files

Hi everybody,

For some reason, when using both `preload-file´ with a bitmap, and firefox, preloading never finishes and the application is never executed.

emrun will look as follows:
```
Preloading file /hello.bmp failed
still waiting on run dependencies:
dependency: fp /hello.bmp
(end of list)
still waiting on run dependencies:
dependency: fp /hello.bmp
(end of list)
still waiting on run dependencies:
dependency: fp /hello.bmp
(end of list)

...
```

Here is a minimal example to trigger this situation:

main.c:
```
int main(int argc, char *argv[]) {
return 0;
}
```

```
emcc --use-preload-plugins --preload-file hello.bmp@/hello.bmp --emrun main.c -o index.html
emrun index.html
```
[hello.bmp.zip](https://github.com/emscripten-core/emscripten/files/6028567/hello.bmp.zip)