TYPO3 Core

Form element definition for FileUpload in typo3/sysext/form/Configuration/Yaml/FormElements/FileUpload.yaml contains

allowedMimeTypes:
  - application/msword
  - application/vnd.openxmlformats-officedocument.wordprocessingml.document
  - application/vnd.oasis.opendocument.text
  - application/pdf

My specific form definition at fileadmin/form_definitions/something.form.yaml overrides settings since I just expect plain-text or CSV uploads:

-
  properties:
    saveToFileMount: '1:/form_uploads/documents/'
    allowedMimeTypes:
      - text/plain
      - text/csv
    elementDescription: ''
  type: FileUpload
  identifier: fileupload-1
  label: Document

However, allowedMimeTypes items are not completely overridden, but merged instead - some mime-types of the initial form element definition are kept - for this example I get:

  - text/plain
  - text/csv
  - application/vnd.oasis.opendocument.text
  - application/pdf

This is caused in TYPO3\CMS\Form\Domain\Model\FormElements\AbstractFormElement::setProperty() which implicitly merges those values. From my point-of-view it's most probably not required to apply initial settings from typo3/sysext/form/Configuration/Yaml/FormElements/FileUpload.yaml at all - however, I did not check/verify that.