[import] Update file content overview after file type change

Summary:

• just like in fileNameChanged() call updateContent() in fileTypeChanged() if necessary
• put those calls in an extra function

Test Plan:

• select an HDF5, NETCDF or ROOT file in the file importer
• choose a different file type
• switch back to the right file type

Reviewers: LabPlot, asemke

Reviewed By: LabPlot, asemke

Subscribers: sgerlach, fkristof, asemke, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D14523