Prevent iPhone 14 from sending pic attachments in .HEIC format

Try Settings > Photos

Scroll to the bottom of the page 

and find "Transfer to Mac or PC"

Make sure that "Automatic" is selected and not "Keep originals", your phone should now convert any image that you end up sharing to another device.

How exactly are you sending these photos? When I send some, I use the photos app where I share a photo, then select the mail icon and enter the email address. When I then send, I get a question about which size photo I want to send Small, Medium, Large or Original (I select Medium). The person I send to does not have an Apple device and the photos arrive without issue.

Yes I see that "Automatic" is already selected and not "Keep originals" within the Settings > Photos section of my iPhone 14.

Are there any other settings I need to change to fix this issue so that when I attach pictures taken on my iPhone within the mail app on the phone these pictures will only be sent in the .JPG and not the .HEIC file format? 

Try setting Formats to Most Compatible in Camera settings. Just to see if that works. 

I've looked in the locations & have tried the steps mentioned above but still haven't seen where to fix this issue.

Where else can I look or what other processes can be used to prevent an iPhone 14 from sending pic attachments in the .HEIC format?

I do not understand why 'most compatible' isn't working for you. I have no issues with emails received with photos attached by everyone I send to. 

>> intermittent issue

Give us as much details as possible about what you mean with this.

>> within Outlook

Again, more details needed. On PC/Mac or other device, what mail provider etc. 

Create a Shortcut that does this:

- Select Photos 

- Convert Photos to JPEG

- Share Converted Image (to mail)

I have such a shortcut to remove exif info and save as JPEG (share). 

This happens on my Windows 10 computer within Outlook 2019 connected to my organization's Office 365 email server.

What are the steps to create a shortcut like you have mentioned above?

>> What are the steps to create a shortcut like you have mentioned above?

By using the Shortcuts app on your iPhone.

- Open the Shortcuts app and press + in the upper right

- Select 'Add Action', 'Apps' and select 'Photos'

- Scroll down and select 'Select Photos'

- At the bottom of the screen close 'Photos' by clicking the X on the right

- Select 'Search for apps and actions'

- Type 'Convert' and select 'Convert Image' under 'Media'

- Select 'Search for apps and actions'

- Type 'Share' and select 'Share' under 'Sharing'

- Select 'Done'

You now have created a shortcut named 'Select Photos', you can use it from Shortcuts or put it on your home screen (you use the share icon at the botton).

To use it, select the newly created shortcut, it will show Photos, select a photo, sharing options appear, select the Mail icon, type or select and email address and press send (icon pointing upwards). Before sending is done, you select the image size, such as 'Medium'. The email that arrives has a .JPEG attached.

Do the images that arrive on your Win 10 machine have .jpg/.jpeg or .heic file extensions?

Trying to determine if your issue is with the phone or Windows.

I use the iPhone mail program on my iPhone 14 to send these pictures.

When I choose the highest resolution option on my iPhone (which is the option I need to use while sending these pictures) these pictures arrive within Outlook 2019 (connected to my organization's Office 365) most often with the .HEIC file extensions.

Then when I download these file attachment pictures within Outlook I don't have any option to change the file extensions for these pictures.

On the other hand if I choose to send these pictures from my iPhone 14 using a lower resolution than the top resolution then these pictures arrive as pictures inline within the email and when I choose the Save as Picture option to save them onto my Windows 10 computer then I can choose which file extension to save them as.

I'm trying to find a way of fixing this issue so that when I send pictures taken on my iPhone 14 with the highest resolution using the iPhone mail program then these pictures will arrive as file attachments in the JPG format (or arrive as pictures inline within the Outlook email on my Windows 10 computer).

So you are still taking photos as .heic on the phone?

If you use Gerwin's suggestion to use "Most compatible" in your camera settings your phone will only use high quality.jpg format and there won't be a need to convert formats at all.

Did you try the Shortcut method you asked me to describe for you?

Yes but within the Select photos portion of these steps I don't see where to  Select 'Search for apps and actions'.

Where can I find the 'Search for apps and actions'?

I'm still in the process of testing this.