apavlov said: 

> Basically, the [current SourceMap v3 spec](https://docs.google.com/a/google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k) defines the "file" property as "An optional name of the generated code that this source map is associated with." 
> So, it's kind of a reverse link from a sourcemap to the file it is referenced in. Not sure whether/how often it is used. IIRC, neither Sass nor LESS produce this field.

Chrome DevTools, at least, doesn't use this field.  According to https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/devtools/front_end/SourceMap.js, it's not accessible by SourceMap clients