How to stop iOS Mail from automatically downloading attachments?
Most attachments in emails are actually remote images. You can stop iOS Mail from downloading remote images as follows:


Go to Settings
Tap on Mail
Under the Messages heading, toggle Load Remote Images off


In terms of stopping other attachments (e.g. MS Word documents etc), there is no such option to prevent downloading these. However, the way it should work is as follows:


When your iPhone is connected to the internet via a cellular network, any email attachments over a certain size will not download automatically.
Instead, for these attachments you would have to manually select them in the body of the email to download them.
When your iPhone is connected to the internet via Wi-Fi, all email attachments will download automatically.


As far as I'm aware there is no way to stop this, although there may be 3rd party email apps that give you this option.

If you're sure that emails are taking up a lot of space on your iPhone, you may need to rethink how regularly you delete older emails. You can also filter emails by attachment and then delete the oldest ones to free up space. Another option is to set up a Mailbox for emails with attachments so it's easier for you to manually manage them.