doc: add posibility of exception

Author: christianberkman

user_guide_src/source/libraries/files.rst

@@ -46,6 +46,8 @@ Returns the size of the uploaded file in bytes:
 
 .. literalinclude:: files/004.php
 
+A RuntimeException will be thrown if the file does not exist or an error occurs.
+
 getSizeByUnit()
 ===============
 
@@ -54,6 +56,8 @@ the results in kilobytes or megabytes, respectively:
 
 .. literalinclude:: files/005.php
 
+A RuntimeException will be thrown if the file does not exist or an error occurs.
+
 getMimeType()
 =============