Import .xlsx and .csv

[bravadomizzou]

Import currently only supports .xls files

When attempting to import .xlsx this message is displayed:
"File has an incorrect mimetype of 'application/zip'"

A similar message is shown when trying .csv files.

[griffbrad]

Add whichever mimetypes you need here:
https://github.com/DeltaSystems/dewdrop/blob/master/Dewdrop/Import/InputFilterFactory.php