Attachments files
A list of all warnings you may encounter when sending an email with attachments.
Here is a list of common messages of error or warning you may get using attachments:

"File is too large"

A file cannot exceed 5 MB in size as an attachment. If you get a warning "File is too large" we can recommend you to lower the size of your file. You can either:
Although Gmail allows uploading file up to 25 MB, the limit allowed during mail merge sending is different. That's why you can send only up to 5MB. As a best practice, we generally recommend that files above 1MB should be sent with a link (here's how to do it). One of the main reasons being that when you mail merge with attachments, it's more likely to trigger spam filters.

"The total size of attachments is too large"

The total sum of your attachments size cannot be more than 5 MB. To lower the size of your files, you can try to compress them, create an archive (.zip) or host them online and link to the folder.

"Too many attachments"

You can add up to 10 attachments to your email merge. If you need to send more files, you can try to create a .zip. But, we recommend you to add a link to a Google Drive folder, which gives a lot more flexibility on the number of files and size.

Spam issues

Most email service providers (such as Gmail) consider that emails with attachments are most likely to be considered as spam. To avoid being flagged as spam, it's best to sent a link to download the files (here's how to do it).

Blocked for security reasons

To protect your account against potential viruses and harmful software, Gmail doesn't allow you to attach certain types of files.
List of blocked file extensions: .ade, .adp, .apk, .appx, .appxbundle, .bat, .bz2, .cab, .chm, .cmd, .com, .cpl, .dll, .dmg, .exe, .gz, .hta, .ins, .isp, .iso, .jar, .js, .jse, .lib, .lnk, .mde, .msc, .msi, .msix, .msixbundle, .msp, .mst, .nsh, .pif, .ps1, .scr, .sct, .shb, .sys, .vb, .vbe, .vbs, .vxd, .wsc, .wsf, .wsh