Fix extension for file download in styles

[NyakudyaA]

Problem

When in edit mode for a layer choosing download layer style results in the extension
.xml appended to the style that is saved on disk.

Solution

Fix and make happy.

NB: Why is it when we download the zipped files we do not get the qml attached in the layers as well ? (#539)

[boney-bun]

@NyakudyaA could you elaborate more?
i can't replicate the issue in the testing and local server.

below is what i did (tested using firefox 61.0.2 on macos):

[NyakudyaA]

hi @boney-bun I only realised that this happens when you are trying to download the style in Safari.

It downloads the style but appends an extension qml.xml

[boney-bun]

hi @NyakudyaA this is odd. usually we got this kind of issues from firefox 😆

ok... i can replicate the issue now.
let me investigate what i can do with it.