To change the file link icons form the content element "File Links", (if you are using css_styled_content) by using Typoscript, we have to give the path to our custom folder, where our custom icons are stored, in our case for example 'fileadmin/templates/icons/'
Typoscript to do so is,
tt_content.uploads.20.renderObj.15.file.import = fileadmin/template/icons/
#to change the file type of the icon, if you want to use png
tt_content.uploads.20.renderObj.15.file.import.wrap = |.png
#change icons globally, if you don’t want to select the different layout in appearance tab other than
#default
tt_content.uploads.20.renderObj.15.stdWrap.if >
If you are using sys extension fluid_style_content, we have to select our own template as explained here.
And then have to give the new icon path in template 'Templates/Uploads.html'.