That fails because the formatter is expecting the incoming parameter to be a path to a file to verify it, rather than the physical file itself.
Instead, you want to use the Input File Values:
[frmFile.Path,Form] – returns the full directory name of the file that is received through the form, from the source machine.
[frmFile.Name,Form] – returns the full file name, including the extension of the file.
[frmFile.NameOnly,Form] – returns the name, without the extension for the file.
[frmFile.Extension,Form] – returns the extensions of the file.
[frmFile.Type,Form] – returns the MIME type (discussed in the appendix)
[frmFile.Length,Form] – returns the physical byte length of the incoming file.
[frmFile.Height,Form] – Returns the vertical height of the file if the file is an Image
[frmFile.Width,Form] – Returns the horizontal width of the file if the file is an Image
In your case you can check to see if the Type starts with Image, or if the Width or Height is not equal to Undefined.