Hi lan,
For the situation, if we open files through Outlook directly, the files will be read-only mode. However, it will quit read-only mode when we save it as to a local path from Outlook.
~~The reason of defaulted read-only mode is caused that the files only temporary files format in Outlook and only be for viewing temporarily.~~Considering format of the attachments in Outlook is .temp, it means the files are temporary files and only for viewing temporarily. So if you would like to make modifications on them, you have to save them as to a local path in proper format like
.docx for saving these changes. If you have more thoughts about this situation, I encourage and suggest you go to UserVoice to vote for
Remove the edit lock on read-only files or to post a new feedback.
Besides, if the files are also locked locally, you could try changing the settings of files by following steps below:
1.Open Finder and navigate to the file on which you want to change permissions. Right-click its icon, and select "Get Info" to enter the file's Info window.
2.Check if the selection "Locked" in General is enabled. If so, disable it and the files will quit Read-only mode.
Regards,
Eli
[Updated by Eli Qian MSFT, 07:40 AM, Jul 25 , 2018 (UTC)]