add more possible extensions

[ggoffy]

No description provided.

[geekwright]

There are potential backward compatibility breaks to add these new extensions -- such as previously invisible things showing up. In general the getImgListAsArray() method is used to supply filenames that can be used directly in an img tag. While tiff is very useful in some situations, it has virtually no support. While better supported, BMP's are generally regarded as a poor choice for web graphics. For now we will stick with the three main types.

To make it easier to produce custom lists using a simple array of file extensions, #623 introduces a new method, XoopsList::getFileListByExtension(). This eliminates the awkward regex maintenance. too.