Click and drag to select multiple files

The idea is that clicking a file and dragging the mouse could add files being touched by the pointer to the selection. This obviously by itself, conflicts with drag and drop, so far two ideas have been proposed to overcome this:

*Clicking the check box, and holding down while dragging the pointer to add files to the selection.

Advantages:
 - To me it's intuitive to think about doing what we already do to select files and just "extend" it to other files.

Disadvantages:
- Missing a click on the small check box may throw away all your selection efforts. And maybe open a lot of stuff at the same time.

*Using a key like control to enter "select mode", so holding this key switches functionality of dragging the mouse (a similar alternative is to make a long click enter this mode).

Advantages:
- We can click anywhere on the file icon without worrying about loosing our selection.

Disadvantage:
- May be difficult to get people to know this can be done until they try and see what a long press, or using control does.

It's important to note that these options are not mutually exclusive and we can implement both of them.