Export atlas as image fail when expression filename contain dot

[MarHoff]

Describe the bug
Under windows 10, atlas export as image fail when filename contain dot "." character while it worked perfectly fine under QGIS 2.18. This is a quite used, accepted and useful character for filenames so I really see this as a regression.

How to Reproduce

0. Download, unzip and open attached demo project
   20190527_bug_path_atlas.zip

1. Go to Layout > "atlas (bugging with all features)"

2. Click on 'preview atlas' and see that all is fine
   

3. Click on export atlas as images (I used png) and choose destination folder
   Notice that export will fail on feature 3 with expected filename "output_foo.bar.png"
   

4. Optionally open Layout > "atlas (working with feature 1&2)"
   Run same steps 2&3 and notice everything work smoothly as feature 3 is excluded in atlas filter.

Desktop environment (please complete the following information):

• OS and version: Winwdows 10

QGIS version

QGIS version	3.6.2-Noosa	QGIS code revision	656500e
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	2.4.1	Running against GDAL/OGR	2.4.1
Compiled against GEOS	3.7.0-CAPI-1.11.0	Running against GEOS	3.7.2-CAPI-1.11.0 b55d2125
PostgreSQL Client Version	9.2.4	SpatiaLite Version	4.3.0
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	5.2.0	Running against PROJ	Rel. 5.2.0, September 15th, 2018

Additional context

Possibly linked to #28829

[elpaso]

@MarHoff thanks for the very detailed bug report, if they were all like this our work would be much much easier!

[MarHoff]

Whoa, thanks a lot for this fast world class support!