Please implement these as derived files in subdirectories. That is; keep the original as is, and allow chained processors in any order with user specified "handles". The handles are subdirectories but filenames are otherwise the same, except for the extensions that may be updated by the processor in a predictable way. So if a user uploads a TeX file, processors can be added for

    /random-file.tex
    "preview"=PNG+Resize(200x400)  - /preview/random-file.png
    "pdf"=TeX-2-PDF(Bicubic=True)  - /pdf/random-file.pdf

Sorry for using a HN thread for this :) And I realize this is a lot of work. Your service looks very promising.