Making a Files: get request will return you the following exportLinks
For a Google Docs File:
{
"exportLinks": {
"application/rtf": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=rtf",
"application/vnd.oasis.opendocument.text": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=odt",
"text/html": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=html",
"application/pdf": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=pdf",
"application/epub+zip": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=epub",
"application/zip": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=zip",
"application/vnd.openxmlformats-officedocument.wordprocessingml.document": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=docx",
"text/plain": "https://docs.google.com/feeds/download/documents/export/Export?id=1qvfrIFTYZ3lL0pkP6VTEG_YdThY0UeB-Oyjfl48sL5o&exportFormat=txt"
}
}
For a Google Sheets File:
{
"exportLinks": {
"application/x-vnd.oasis.opendocument.spreadsheet": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=ods",
"text/tab-separated-values": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=tsv",
"application/pdf": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=pdf",
"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=xlsx",
"text/csv": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=csv",
"application/zip": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=zip",
"application/vnd.oasis.opendocument.spreadsheet": "https://docs.google.com/spreadsheets/export?id=1wGIfU1L9__Tt1JIxfkCYqUiUH1RSIt3P7KsMsuVUYIc&exportFormat=ods"
}
}
In other words: There is no inbuilt functionality to export a Google Sheets file to HTML
This is the reason why you can email a Docs files from the UI as HTML, but not a Google Sheets.
Conclusion:
You need to build an html table yourself to get the sheet content into your email body.
Here is a sample of how you can do it.
Alternatively - consider converting the whole sheet content to an image, since images can be isnerted into the email body.