You cannot add a text file as an attachment to a child object when you select the History enabled check box on the child object. 

For example, this issue occurs when you complete steps similar to the following:

1. Create a parent document and a child document.
2. Add an image file to the child document.
3. Add a text file to the child document.

As a result, the following payload from the Network Trace for adding the attachment occurs:

Text File
{
    "id": "85a90014-9b34-44df-a224-1dbecc366a75",
    "name": "test.text",
    "location": "/files/files/85a90014-9b34-44df-a224-1dbecc366a75/content",
    "type": "text/plain",
    "size": 54,
    "description": "automation text test",
    "category": "Attachment"
}

Image File
{
    "id": "eb5eae54-5d23-4b58-afc1-ebdc8bd863d8",
    "name": "test.jpg",
    "location": "/files/files/eb5eae54-5d23-4b58-afc1-ebdc8bd863d8/content",
    "type": "image/jpeg",
    "size": 54,
    "description": "automation image test",
    "category": "Attachment"
}

{
    "httpStatusCode": 500,
    "errorCode": "DH9000",
    "message": "A system internal error occurred. Please contact support team with details.",
    "version": 1
}

There is no workaround for this issue.