Submitting charset when adding text file

[lrosenth]

In scripts/convert_from_binaries.lua we expect the user provided mimetype to contain also the charset (separated by a ;).

If the charset is not submitted, Sipi's answer JSON does not contain a charset member which causes a problem in Knora: it cannot be converted to a SipiTextResponse

Proposed solution: Sipi should reject a request without charset (or Knora should check before sending the request to Sipi)