Send email to only interested colleagues when an attachment is added

Hi Sprinx,
There is no direct trigger to detect new attachment. But you can use a workaround like, add JSON formatting on List column to trigger workflow directly from list. So when new attachment is added to list item, you can click button to run workflow that will send email to listed users.
Heres a sample JSON formatting I used to trigger workflow from List
NOTE: Use "for a selected item" trigger in workflow.

{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "button",
"customRowAction": {
"action": "executeFlow",
"actionParams": "{\"id\": \"YOUR WORKFLOW ID\"}"
},
"attributes": {
"class": "ms-fontColor-themePrimary ms-fontColor-themeDarker--hover"
},
"style": {
"border": "none",
"background-color": "transparent",
"cursor": "pointer"
},
"children": [
{
"elmType": "span",
"attributes": {
"iconName": "Mail"
},
"style": {
"padding-right": "16px"
}
}
]
}