related to MDL-16359, I am leveraging the ability to go from icon types (roughly equivalent to portfolio formats for now) to supported mimetypes for any given format.

i've tested this with the flickr plugin (which i've changed to support image and video formats) and it's working nice.

also changed docs to add notes about supportedformats for single file export and mime detection:

http://docs.moodle.org/en/Development:Adding_a_Portfolio_Button_to_a_page#supported_formats_.28static.29_.28read_this_even_if_you_don.27t_want_to_override_this_function.2C_it.27s_important.29

and changed all logical places to be more specific about supported formats.

also related to MDL-16124