Transformations that access a file's data can only be used first in a pipeline

[agsimmons]

For example, if you change the extension of an audio file from ".mp3" to ".MP3" as the first transformation in a pipeline, then try to use the music_tags transformation, the audio file won't be found, as it is passed the modified file sequence before the filenames have been modified.

This might require passing around a file sequence class instead of a list of paths, where the original path and the currently modified paths can be accessed by a transformation at will.