cut out the name of a document

I think you need to understand what index() does -- when you give it a number as the second parameter, it attempts to return a corresponding array position from the source data.  Since a text string is really just an array of characters, it will return you the letter at that position of the string.  Note this isn't really a conventional use of index() (and not every function will treat a string as an array of characters), but that's what it's doing here, and from what I can tell it's totally unrelated to what you're really after.

I believe you might have some success using the "fileNames" parameter of FileUploadField and your expression rule.  Personally, I'd opt instead to just use a user-facing validation similar to the following:

a!localVariables(
  local!file: null(),

  a!fileUploadField(
    saveInto: local!file,
    value: local!file,
    maxSelections: 1,
    validations: {
      if(
        len(fv!files.name) > 5,
        "filename too long (5 characters for this example)",
        null()
      )
    }
  )
)

However I assume (but haven't tried per se) that you could instead use the "fileNames" parameter as I mentioned to auto-truncate the filename - note that, as far as I can tell, the change doesn't happen until Form Submission time, so you'll need to experiment with it some.

a!localVariables(
  local!file: null(),

  a!fileUploadField(
    saveInto: local!file,
    value: local!file,
    maxSelections: 1,
    
    fileNames: left(fv!file.name, 5)  /* insert your own length here */
    
    /*validations: {
      if(
        len(fv!files.name) > 5,
        "filename too long (5 characters for this example)",
        null()
      )
    }*/
  )
)

Really useful... Thank you so much!

Really useful... Thank you so much!