You can favor one file type over another by appending contenttype:filetype in a search term
eg: test contenttype:pdf
will yield results with pdf files on top.
You can also search level boosting to favor the file type as below
eg: test contenttype:pdf^10
If you need html files to be displayed on top the search should be as follows
If you need jpg files to have a greater preference the search should be as follows
To learn about Search in SearchBlox read: Search Syntax and Semantics in SearchBlox