Why is nuget not adding my file? netcoreapp2.2

I have this nuspec file:

<?xml version="1.0"?>
<package xmlns="http://schemas.microsoft.com/packaging/2013/05/nuspec.xsd">
  <metadata>
    <id>test</id>
    <version>1.0.0</version>
    <authors>Dejana Tesic, Rajeev Dey</authors>
    <owners>dejanatc, rjdey</owners>
    <projectUrl>http://github.com/contoso/UsefulStuff</projectUrl>
    <license type="expression">Apache-2.0</license>
    <iconUrl>http://github.com/contoso/UsefulStuff/nuget_icon.png</iconUrl>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <releaseNotes>Bug fixes and performance improvements</releaseNotes>
    <description>Core utility functions for web applications</description>
    <copyright>Copyright ©2016 Contoso Corporation</copyright>
    <tags>web utility http json url parsing</tags>
  </metadata>
  <files>
    <file src="a.txt" target="content" />
  </files>
</package>

when I install this package, a.txt is not added to the solution.

What am I missing here?