In Workflow settings --> Conditional Start option gives you the current and previous values of a column, here you can check current and previous values of a column/field and then decided whether to run the workflow or not when an item is edited (if you want to trigger the workflow on item change).

 

So you can conditionally start the workflow if a certain column has a value now that is different to the previous value.

 

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Inside the workflow, choose Workflow Settings from the ribbon bar.

Under Workflow Options, set "Start when items are created" to No, and "Start when items are modified" to Conditional.

In the pop up box, choose "Attachments (Previous value)", Not Equal, check the box "Compare to field?", and finally "Attachments". Click Save and republish the workflow.

The workflow will only initiate when that one field is changed. Note: putting too many or too complex a condition will result in longer save times, as they are evaluated during the save process.

@blythesr is 100% spot on, just wanted to add step by step instructions.

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