RELEASE 2.2.3 OR ABOVE
Use 'editor.import.fileFiledName', 'editor.upload.image.fileFiledName', 'editor.upload.video.fileFiledName' and 'editor.upload.file.fileFiledName' keys to set fieldName for formData in upload and import.
synapeditor.config.js
{
'editor.import.fileFieldName': 'file',
'editor.upload.image.fileFieldName': 'file',
'editor.upload.video.fileFieldName': 'file',
'editor.upload.file.fileFieldName': 'file'
}