Here is a list of common messages of error or warning you may get using attachments:
A file cannot exceed 4 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:
compress your files, using an online service; or
host your files on a Google Drive (or others file hosting solution).
The total sum of your attachments size cannot be more than 4 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.
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.
To protect your account against potential viruses and harmful software, Gmail doesn't allow you to attach certain types of files, including:
.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