Fix mimeType loss during file creation or update.

Summary:
LibKGAPI allows specifying the mimeType of an upload via the associated KGAPI2::Drive::File.
This is required in uploads where the data isn't sufficient in determining the mime-type (i.e. docx files as reported in BUG 401151).

BUG: 401151

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: dvratil, KIO GDrive

Tags: KIO GDrive

Differential Revision: https://phabricator.kde.org/D22628