Missing file extension causes AttributeError in 'TilingThread'

[cvgh9]

QGIS version: 3.22.4-Białowieża
OS: Ubuntu 22.04.2 LTS
Python version: 3.10.3
QTiles version: 1.7.2

Entering an output filename using the browser does not add the file extension. If not entered manually an Attribute error occurs in TilingThread.py at line 131: if self.mode == 'DIR':. Without a file extension mode is not set in line 80 - 87 for non-directory options.

if self.output.isDir():
    self.mode = 'DIR'
elif self.output.suffix().lower() == "zip":
    self.mode = 'ZIP'
elif self.output.suffix().lower() == "ngrc":
    self.mode = 'NGM'
elif self.output.suffix().lower() == 'mbtiles':
    self.mode = 'MBTILES'

Possibly the cause of #12

Does not occur on Windows 10, the file manager adds the file extension automatically.