diff --git a/docs/collector.textfile.md b/docs/collector.textfile.md
index 41e21ad1..a1b7d3e2 100644
--- a/docs/collector.textfile.md
+++ b/docs/collector.textfile.md
@@ -12,7 +12,7 @@ Enabled by default? | Yes
 
 ### `--collector.textfile.directory`
 
-The directory containing the files to be ingested.
+The directory containing the files to be ingested. Only files with the extension `.prom` are read.
 
 Default value: `C:\Program Files\wmi_exporter\textfile_inputs`