ValuePosition appears to be a custom function. Perhaps it is not working as expected. Have you checked to see that the correct branch of the If - Else IF tree is executing with each selected file type?
I'd use this expression to check and see if $FileType is a member of a particular value list's list of values:
Not IsEmpty ( FilterValues ( ValueListItems ( Get ( FileName ) ; "ValueListNameInQuotes" ) ) )
No custom function needed.
My problem is not with the logic of my If statement, but with inserting files into a container field. I mentioned above that it was throwing an error that the file cannot be found. This is happening on the Insert Picture/PDF/Etc. steps. I have included the error below to illustrate. Thank you for taking the time to try to help, however.