Mail (Yosemite): Add attachments
When you attach images or PDF documents, you can mark them up in Mail before you send your message.
Add photos and other files
Drag files from Finder, iPhoto, or another app into your message. Or click the Attach button 
in the toolbar of the message window, then select a file. For photos, you can click the Photo Browser button 
in the toolbar, then drag a photo into your message.
By default, Mail inserts images in their actual size. If different sizes are available, choose one from the pop-up menu located on the right side of the message header.
Send large attachments using Mail Drop
You can use Mail Drop to send files that exceed the maximum size allowed by the provider of your email account. Mail Drop uploads the large attachments to iCloud, where they’re encrypted and stored for up to 30 days.
If you have an iCloud account and you’re signed in to iCloud when you click Send, Mail automatically sends the attachments using Mail Drop. Mail Drop attachments don’t count against your iCloud storage.
If you don’t have an iCloud account, or if you’re not signed in, Mail asks you whether to use Mail Drop (select “Don’t ask again for this account” to always use Mail Drop).
If a recipient uses Mail in OS X Yosemite, the attachments are automatically downloaded and included in your message just like any other attachment. For other recipients, your message includes links for downloading the attachments and their expiration date.
You can turn Mail Drop on or off for an account. Choose Mail > Preferences, click Accounts, select your account, click Advanced, then select or deselect “Send large attachments with Mail Drop.”
Put attachments at the end of messages
For the current message: Choose Edit > Attachments > Insert Attachments at End of Message.
For all messages: From the main Mail window, choose Edit > Attachments > Always Insert Attachments at End of Message.
Include or exclude attachments in replies
Include original attachments in a reply: In the toolbar of the message window, click the Include Attachment button 
or choose Edit > Attachments > Include Original Attachments in Reply.
Exclude original attachments in a reply: In the toolbar of the message window, click the Exclude Attachment button 
or choose Message > Remove Attachments.
Include original attachments in all replies: From the mail Mail window, choose Edit > Attachments > Include Original Attachments in Reply.
Send attachments to Windows users
Follow these suggestions:
Make sure you send Windows-friendly attachments. To do so for all messages, from the main Mail window choose Edit > Attachments, then make sure Always Send Windows-Friendly Attachments is selected. For a specific message, click the Attach button in the toolbar of the message window, then make sure Send Windows-Friendly Attachments is selected.
Instead of sending a Pages, Keynote, or Numbers document, export the document as a PDF, then send the PDF.
Use filename extensions (such as .docx for a Microsoft Word document).
If the recipient sees two attachments (such as “MyFile” and “._MyFile”), the file with the underscore (such as “._MyFile”) can be ignored.
If the message size located on the left side of the message header is red, the attachments are causing your message to exceed size limits set by your email account provider. Try reducing the number or size of attachments or, if available for your email account, use Mail Drop.