File Attachment Bug

Hi, I have been struggling with this Outlook bug for months now where every once in a while I am unable to attach any files, even small pdf files, to brand new e-mail chains. I get an error telling me to "remove inline files of large dimensions from this message thread" even though the thread is brand new! I tried all of the workarounds, including switching message format to html (which it was already set to), signing in and out again, deleting all of my cookies and cached files, deleting all and any saved drafts (which I do all the time anyways), and using incognito mode to access Outlook. I do not understand why this happens and frankly I find it unbelievable that this has been going on for so long with no fixes! If anyone has any other advice/fixes please let me know. Thanks!