fileupload

One basic thing - To upload a document in Appian, it has to be uploaded in a User input task or a Process start form. So you'll anyway have to create an interface and map it in the RI. But for this, you just need a RI of document type (list) and map it as the value and save into in the file upload field. 

a!formLayout(
  label: "Upload File",
  contents: {
    a!sectionLayout(
      contents: {
        a!fileUploadField(
          label: "File Upload",
          instructions: "Only xlsx and csv file allowed",
          labelPosition: "ABOVE",
          target: cons!MP_UPLOADED_FILE,
          fileNames: substitute(fv!file.name, " ","_"),
          fileDescriptions: fv!file.name &" "&" uploaded",
          maxSelections: 2,
          value: ri!document,
          saveInto: {ri!document},
          required: true,
          validations: a!localVariables(
            local!invalidExtension: difference(upper(fv!files.extension),{"XLSX","CSV"}),
            {
            if(
              local!invalidExtension>0,
              "Attachments must be xlsx or csv. Remove: " &
              index(fv!files, "name", wherecontains(local!invalidExtension, upper(fv!files.extension)),{}),
              ""
            )
          }),
          buttonStyle: "SECONDARY"
        )
      }
    ),
    a!sectionLayout(
      label: "",
      contents: {}
    )
  },
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidget(
        label: "Submit",
        submit: true,
        style: "PRIMARY"
      )
    },
    secondaryButtons: {
      a!buttonWidget(
        label: "Cancel",
        value: true,
        saveInto: ri!cancel,
        submit: true,
        style: "NORMAL",
        validate: false
      )
    }
  )
)

You can try this code, you can configure a rule input for document and make it array. The target is the constant for folder