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I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

HINT:

You may need to change Mail to com.apple.mail in the path. Background is the following: You may see multiple folders called Mail in ~/Library/Containers/. You may think, these are all duplicates, but they aren't. Mail isn't their real name. You can check the real name for each of them by opening the information window with ⌘+i. Under the section Name & Extension you will see the real name, like com.apple.mail, com.apple.mail.MailQuickLookExtension or com.apple.MailShareExtension.

I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

HINT:

You may need to change Mail to com.apple.mail in the path. Background is the following: You may see multiple folders called Mail in ~/Library/Containers/. You may think, these are all duplicates, but they aren't. Mail isn't their real name. You can check the real name for each of them by opening the information window with ⌘+i. Under the section Name & Extension you will see the real name, like com.apple.mail, com.apple.mail.MailQuickLookExtension or com.apple.MailShareExtension.

EDIT: If you're using macOS Ventura check comment of Aussie Geek.

I was able to find the solution myself.

You have to do the following:

I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

HINT:

You may need to change Mail to com.apple.mail in the path. Background is the following: You may see multiple folders called Mail in ~/Library/Containers/. You may think, these are all duplicates, but they aren't. Mail isn't their real name. You can check the real name for each of them by opening the information window with ⌘+i. Under the section Name & Extension you will see the real name, like com.apple.mail, com.apple.mail.MailQuickLookExtension or com.apple.MailShareExtension.

I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

HINT:

You may need to change Mail to com.apple.mail in the path. Background is the following: You may see multiple folders called Mail in ~/Library/Containers/. You may think, these are all duplicates, but they aren't. Mail isn't their real name. You can check the real name for each of them by opening the information window with ⌘+i. Under the section Name & Extension you will see the real name, like com.apple.mail, com.apple.mail.MailQuickLookExtension or com.apple.MailShareExtension.

I was able to find the solution myself.

You have to do the following:

I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

I was able to find the solution myself.

You have to do the following:

I was able to find the solution myself.

You have to do the following:

1. Quit the Mail app
2. Go to ~/Library/Containers/Mail/Data/Library/Preferences/com.apple.mail.plist
3. Open com.apple.mail.plist file, e.g. in Xcode
4. Remove the parameter MFAttachmentBehaviorKey
5. Safe the file and start Mail
6. Done!

HINT:

You may need to change Mail to com.apple.mail in the path. Background is the following: You may see multiple folders called Mail in ~/Library/Containers/. You may think, these are all duplicates, but they aren't. Mail isn't their real name. You can check the real name for each of them by opening the information window with ⌘+i. Under the section Name & Extension you will see the real name, like com.apple.mail, com.apple.mail.MailQuickLookExtension or com.apple.MailShareExtension.