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Field Value block outputs a value of a field without extras.
In the case of wrapping a URL in a link, instead use a Single Item block, which allows setting a template, such as:
<a href="https://projectdmc.org/support/topic/block-editor-file-field/{@file_field_name._src}">
{@file_field_name.post_title}
</a>…which would output a link to the URL of the original file, using the name of the file as the text for the link.