Acepting only two types of extensions

The requirement to have it spit out the list of files with invalid extensions makes it a little convoluted - a simpler validation message would be much easier to code and understand, FWIW.

Anyway -- first off, there's a mistake in your index() function - you have 4 parameters (fv!files, then name, then your wherecontains(), then an empty set); so this wouldn't work under any circumstances.  The correct way of writing this would be index(fv!files.name, ...), which I believe is safe because the only time the validation message would attempt to evaluate, fv!files would have to be populated (and therefore have the .name property).

Overall, though, my suggestion for you (after fixing the index() function) is to front-load a bit more of your logic into local variables so that they're understandable - in particular, the wherecontains() call.  If we calculate the indices of the bad extensions ahead of time, the later index() function becomes a lot more easy to understand.  For example:

a!localVariables(
  local!uploadedDoc,
  
  a!fileUploadField(
    value: local!uploadedDoc,
    saveInto: local!uploadedDoc,
    validations: a!localVariables(
      local!allowedExtensions: {"png", "jpg"},

      local!invalidExtensions: difference(upper(fv!files.extension), { "PNG", "JPG" }),
      
      local!badExtensionIndices: wherecontains(
        local!invalidExtensions,
        upper(fv!files.extension)
      ),

      if(
        not(rule!APN_isEmpty(local!invalidExtensions)), 
        "Attachments must be images. Remove: " &
        index(
          fv!files.name, /* this was the main error before */
          local!badExtensionIndices,
          {}
        ), 
        null()
      )
    )
  )
)

Let me know if you have any questions.

The requirement to have it spit out the list of files with invalid extensions makes it a little convoluted - a simpler validation message would be much easier to code and understand, FWIW.

Anyway -- first off, there's a mistake in your index() function - you have 4 parameters (fv!files, then name, then your wherecontains(), then an empty set); so this wouldn't work under any circumstances.  The correct way of writing this would be index(fv!files.name, ...), which I believe is safe because the only time the validation message would attempt to evaluate, fv!files would have to be populated (and therefore have the .name property).

Overall, though, my suggestion for you (after fixing the index() function) is to front-load a bit more of your logic into local variables so that they're understandable - in particular, the wherecontains() call.  If we calculate the indices of the bad extensions ahead of time, the later index() function becomes a lot more easy to understand.  For example:

a!localVariables(
  local!uploadedDoc,
  
  a!fileUploadField(
    value: local!uploadedDoc,
    saveInto: local!uploadedDoc,
    validations: a!localVariables(
      local!allowedExtensions: {"png", "jpg"},

      local!invalidExtensions: difference(upper(fv!files.extension), { "PNG", "JPG" }),
      
      local!badExtensionIndices: wherecontains(
        local!invalidExtensions,
        upper(fv!files.extension)
      ),

      if(
        not(rule!APN_isEmpty(local!invalidExtensions)), 
        "Attachments must be images. Remove: " &
        index(
          fv!files.name, /* this was the main error before */
          local!badExtensionIndices,
          {}
        ), 
        null()
      )
    )
  )
)

Let me know if you have any questions.

Double-checking your original code, I can confirm that just fixing the index() call seems to have been enough to make it work.  For what it's worth

I see, that does make more sense. I got it to work with your logic example you provided. One more thing, do you happen to know how to get the upload button to accept multiple files. Before adding the validation, I was able to upload multiple files. Now, it only lets me upload one. 

The only thing I know of that would do that is setting a value for the maxSelections parameter.  For example, my original code snippet above allows me to upload multiple (i just double checked).  Did you change anything else?