files and directories - Manipulate and FileNameSetter don't want to play nice together

In a Manipulate, one can specific the type of an individual control like so:

Manipulate[v, {v, Red, ColorSetter}]

I tried to do the same with FileNameSetter, i.e.

Manipulate[v, {v, "no file chosen", FileNameSetter}]

but that doesn't work, and the control is a slider. I can work around the issue like so:

Manipulate[{v, FileNameSetter[Dynamic[v]]}, {v, "no file chosen"}, 
  ControlType -> None]

but I would rather understand why my second example above doesn’t behave as expected. Do you have any idea?

Answer

Manipulate does not directly support a control type called "FileNameSetter", but fortunately it is possible to use custom controls (both in Manipulate and other functions), as described e.g. in the Control documentation:

{u,func}        an arbitrary control from a function

The trick to getting it working is that func must be a pure function. This is mentioned in the Advanced Manipulate Tutorial, in the section showing how to build a custom slider.

So, somewhat unusually, Control[{v, FileNameSetter}] will not work, but Control[{v, FileNameSetter[##]&}] will. The equivalent construction using Manipulate would be:

Manipulate[v, {{v, "none"}, FileNameSetter[##] &}]

(Of course things would be much simpler if Manipulate would just support FileNameSetter directly as it does ColorSetter)