fix RegExp FileExtension

reFileExtension had a problem caused false replacing of extensions 
(for examble extension "ubi" replaced ubi in ubiquity and left extension "bin")

e.g. file:
gluon-comm-0.1.2-stable.0-20190722-ubiquiti-unifi-ac-mesh-sysupgrade.bin

would have been (wrong):
quiti-unifi-ac-mesh-.bin.jpg

instead of (correct): 
ubiquiti-unifi-ac-mesh.jpg

should fix (at least parts) of #92 


not sure if the first point in line 123 should also be escape 
RegExp(/.(bin|img\.gz|img|tar|ubi)$/);
RegExp(/\.(bin|img\.gz|img|tar|ubi)$/);