Shouldnt be a problem, can i see the file texture you are using.

basically you want to use the facing ratio to modify the reflectivity of your file texture. To do this you can use a layer texture node.

Color entery list referes to the value on a point of a ramp, since a file texture isnt a ramp and sources data from an external file the same process does not apply.

The layered texture node would allow your facing ratio ramp to influence the values of your file texture using blending methods like in photoshop.

If your using the photoshop file just for the reflectivity then i would work in color and then use an rgb> luminance node to convert the values from a vector to a float so it will be compatable with the reflectivity attribute of your surface shader.

Maya does not discriminte between what channels you are using for transparency etc. It does not matter wether you hold your red color information in red or your diffuse or transparency information in the alpha channel. I did a similar network for a project i did:



Note how the general transparency is defined by the cloud inputting into ramp 1, and then modified with facing ratio using a layered texture. Sorry for all the extra information, i used the breakdown for someone else in the past.