file-upload: plain text files not handled correctly

[anne17]

Describe the bug
The info box states that one can upload plain text files. Upon upload those files are stored with the xml extension (even if the original file had another extension) but their content is not converted into xml (no root tag added, no escaping of special characters is done). The pipeline then crashes if the content contains unescaped special characters.

Info for reproducing the bug

1. Sparv version used: v3
2. URL called: https://spraakbanken.gu.se/sparv/#input=file&lang=sv&language=sv
3. hash of your build: 5dddcaeccdbeda62e1349e49073a0568fa68d082-f (saved in demo.spraakdata.gu.se/export/htdocs/anne/Sparv%20crashtests)