Hi @MiniMe_83 :
I've made a test for your reference:
1\I assume there is a library-picc
2\I assume there is a list -ListA
3\I assume the form is a new form
Step 1: Create a gallery and set it's items property to:
picc
Step 2: Create a flow-GetFileContent(to get the file content):

Reference Expression:
data:@{outputs('Get_file_content_using_path')?['body']['$content-type']};base64,@{outputs('Get_file_content_using_path')?['body']['$content']}
Step 3:Add the flow into my app and add a button into the gallery and set it's OnSelelct property to
Set(thefile,GetFileContent.Run("/"&ThisItem.'Full Path').content);
Collect(TheAttachmentCollection,{DisplayName:ThisItem.'File name with extension',AbsoluteUri:"", Id: "00000000-0000-0000-0000-000000000000",Value:thefile})
Step 4:Set the attachment control's Items property to:
TheAttachmentCollection
and set it's property to

Step 5: Submit the form
SubmitForm(Form1);ResetForm(Form1);Clear(TheAttachmentCollection)
The Result:

Best Regards,
Bof