Added docs entry for validfiledata

Changelog: None
Ticket: ENT-8193
Signed-off-by: Victor Moene <victor.moene@northern.tech>

Author: victormlg

content/reference/functions/validfiledata.markdown

@@ -0,0 +1,33 @@
+---
+layout: default
+title: validfiledata
+aliases:
+  - "/reference-functions-validfiledata.html"
+---
+
+{{< CFEngine_function_prototype(filename, filetype) >}}
+
+**Description:** Parses CSV, JSON, or YAML data from file `filename`
+and returns `true` if the content is valid and `false` otherwise.
+
+When `filetype` is `auto`, the file type is guessed from the extension
+(ignoring case): `.csv` means CSV; `.json` means JSON; `.yaml` or `.yml` means
+YAML. If the file doesn't match any of those names, JSON is used.
+
+{{< CFEngine_function_attributes(filename, filetype) >}}
+
+**Example:**
+
+```cf3
+bundle agent main
+{
+  vars:
+    "content"
+      data => readdata("/tmp/a.json", "auto"),
+      if => validfiledata("/tmp/a.json", "auto");
+}
+```
+
+**See also:** [`data_expand()`][data_expand], `readdata()`, `readcsv()`, `readyaml()`, `readjson()`, `readenvfile()`, `validdata()`, `data` documentation.
+
+**History:** Was introduced in 3.29.0.