Improve handling encodings

Html and text files are not fixed to be UTF-8. The encoding is now
detected, which may not work for all files. Default/fallback will be
utf-8.

There is still a problem with mails that contain html parts not in
utf8 encoding. The mail text is always returned as a string and the
original encoding is lost. Then the html is stored using utf-8 bytes,
but wkhtmltopdf reads it using latin1. It seems that the `--encoding`
setting doesn't override encoding provided by the document.

modules/restserver/src/main/scala/docspell/restserver/conv/Conversions.scala

@@ -448,7 +448,7 @@ trait Conversions {
   // MIME Type
 
   def fromContentType(header: `Content-Type`): MimeType =
-    MimeType(header.mediaType.mainType, header.mediaType.subType)
+    MimeType(header.mediaType.mainType, header.mediaType.subType, header.mediaType.extensions)
 }
 
 object Conversions extends Conversions {