Currently, under 'Space > List Attachments', the option 'Filter By File Extension' is case-sensitive.

This means that if you search for files with the extension 'png', and the physical filenames actually end in 'PNG', then the search will return nothing.

Also ... if, for example, some attachments in the space have the extension 'png' and others have the extension 'PNG', then searching for either 'png' or "PNG' returns an incomplete list.