Is there a conventional format for an alpha mask?

I'm a programmer, not a graphic designer. I'm writing a custom View for Android that requires the ability to apply an alpha mask to an image. I want to make it easy for graphic designers to produce media for this software.

Is there any standard or conventional file format for an alpha mask? In other words, if I were to ask a graphic designer to give me an alpha mask, what would it look like? The most common example I find on the web is a bitmap with black indicating complete transparency and white indicating complete opacity. Is that a format that graphic designers would be familiar with?

If the answer differs by application, I'm most interested in what would be familiar to a designer using Photoshop.