fixed: loading of images from urls which don't have an extension...

... and thus we weren't able to determine the mimetype. This will fix #14781.
The downside is that we had the mimetype in CBaseTexture *CTextureCacheJob::LoadImage already but won't pass it along. But I don't think this is much overhead and we should check for the mimetype there in any case.

Additional I removed an unused method.