fix(rust): add extension to the "filename" in the disposition header

[imobachgs]

Problem

When downloading the logs, Chrom(ium|e) does not add an extension to the file name. In Firefox it seems to work just fine. See #2141.

Solution

Set the extension in the filename so Chromium uses the correct name.

Testing

• Tested manually

Screenshots

If the fix affects the UI attach some screenshots here.

[coveralls]

Pull Request Test Coverage Report for Build 13812207531

Details

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-16.7%) to 72.477%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rust/agama-server/src/manager/web.rs	0	6	0.0%

Totals
Change from base Build 13810759906:	-16.7%
Covered Lines:	20016
Relevant Lines:	27617

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💛 - Coveralls