Hi @DemoMan,
Could you please share a screenshot of your flow's configuration?
Could you please show more details about your SharePoint list?
Further, do you want to put all attachment names into a Multiple lines of text column of your SharePoint list?
I have created a SharePoint list on my side and the data structure of it as below:
Note: The Subject and Body column are both Single line of text type column, the Received Time column is a Date and Time type column, the From and To column are both Person or Group type column, the AttachmentNames column is a Multiple lines of text type column.
I have made a test on my side and please take a try with the following workaround:
- Add a "When a new email arrives" trigger, Has Attachment field and Include Attachments field are both set to Yes.
- Add a Variables->"Initialize variable" action, Name set to AttachmentNames, Type set to String and Value set to empty.
- Add a "Html to text" action, Content field set to Body dynamic content of the trigger.
- Add a "Apply to each" action, input parameter set to output of the Attachments dynamic content of the trigger.
- Within "Apply to each" action, add a "Compose" action, Inputs field set to follwoing formula:
concat(item()?['Name'],'</br>',variables('AttachmentNames'))
Add a "Set variable" action, Name choose AttachmentNames, Value set to output of the "Compose" action.
- Under "Apply to each" action, add a "Create item" action, the AttachmentNames field set to AttachmentNames variable.
Image reference:

The flow works successfully as below:

More details about using expression in flow actions, please check the following article:
Use expression in flow actions
Best regards,
Kris