First, files are be converted to PDF for archiving. It is also easier
to create a preview. This is done via the `ConvertPdf` processing
task (which is not yet implemented).
Text extraction then tries first with the original file. If that
fails, OCR is done on the (potentially) converted pdf file.
To not loose information of the original file, it is saved using the
table `attachment_source`. If the original file is already a pdf, or
the conversion did not succeed, the `attachment` and
`attachment_source` record point to the same file.
